--------“নুরে মুহাম্মদি” এবং The Cosmic Microwave Background (CMB)--- নবীর নূর বা নুরে মুহাম্মাদী, নিয়ে বিজ্ঞ মহলে আনেক ধরনের মতবাদ বা কথা প্রাচলিত আছে , বা বলতে পারেন নূর শব্দের বিভিন্ন ব্যাবহার, যা মটামটি কয়েকটা পয়েন্ট এ নিম্নে দিলাম। ১। আল্লাহ্ কোরানে নবী (স) কে নূর আক্ষাইত করেছেন। যেমন ”কাদ জায়াকুম মিনাল্লাহে নূরুন ওয়া কিতাবুম্ মুবীন” (৫:১৫)। অর্থ: ”নিশ্চয় তোমাদের কাছে এসেছেন আল্লাহর পক্ষ থেকে এক নূর (আলো, জ্যোতি) এবং স্পষ্ট কেতাব (আল্ কুরআন)।” -- এই আয়াতে ব্যাপারে সকল মুহাদ্দিস অ তাফসির কাররা এক মত যে এই আয়াতে নূর বলাতে নবী পাক (স) কেই বুজান হয়েছে, তবে অর্থ বিভিন্ন জনে ভিন্ন করেছেন কেহ বলেছেন, নবুওতি কে বুজান হয়েছেন, কেহ বলেছেন ঐশী গিয়ান ইতায়াদি ইত্যাদি। কোরানে আর এক টা আয়াত আছে যেখানে আল্লাহতা’লা এরশাদ ফরমান: ”তাঁর (আল্লাহর) আলোর (নূরের) উপমা হলো এমনই যেমন একটা দীপাধার, যার মধ্যে রয়েছে প্রদীপ। ওই প্রদীপ একটা ফানুসের মধ্যে স্থাপিত। ওই ফানুস যেন একটি নক্ষত্র, মুক্তার মতো উজ্জ্বল হয় বরকতময় বৃক্ষ যায়তুন দ্বারা, যা না প্রাচ্যের, না প্রতীচ্যের; এর নিকটবর্তী যে, সেটার তেল প্রজ্জ্বলিত হয়ে ওঠবে যদিও আগুন সেটাকে স্পর্শ না করে; আলোর (নূরের) ওপর আলো (নূর)।” (আল্ কুরআন, ২৪:৩৫) --- এই আয়াতের ব্যাখ্যায় “নুরের উপর নূর” কে আনেকে প্রথম নূর আল্লাহর নূর এবং দিতিও নূর কে নবীর নূর বুজাতে চান। -- কিন্তু এই ক্ষেত্রে দিতিও নূর, নবীর নূর টা ঠিক নয় কারন এই আয়াতে আল্লাহ্ কেবন নিজের নূর বিসয়ে বর্ণনা অ ইয়পমা দিয়েছেন, নবীর নুরের এখানে কোন কথা আসে না। ইহা ভিন্ন জিনিস। ২। যেহেতু আল্লাহ্ আমাদের নবী (স) কে নূর আক্ষাইত করেছেন। সেই হেতু অনেক আমাদের নবী (স) কেবল মাত্র নুরের তৈরি, মাটির নয় বলেন। আনেকে আবার এটা মানেন না, বলতে চান কেবল ই মাটির তৈরি। অথচ কোন বস্তুই একক জিনিসের তৈরি নয়। ৩ আরেকটা নুরের উল্লেখ আছে যা আমাদের প্রিয় নবী (স) আল্লাহর কাছে দোওয়ার মাধ্যমে চাইতেন। যেমন ক। হযরত ইবনে আব্বাস (রা:) বর্ণনা করেন যে রাসূলে খোদা (দ:) সেজদারত অবস্থায় আরয করেন: ”এয়া আল্লাহ! আপনি আমার কলবে (অন্তরে) নূর (আলো/জ্যোতি) স্থাপন করুন; আরও স্থাপন করুন আমার শ্রবণশক্তি ও দৃষ্টিশক্তিতে, আমার ডানে ও বামে, আমার সামনে ও পেছনে, ওপরে ও নিচে; আমার জন্যে নূর সৃষ্টি করুন।” অথবা তিনি বলেন, “আমাকে নূর (আলো) করুন।” হযরত সালামা (রা:) বলেন, “আমি কুরাইব (রা:)-এর দেখা পাই এবং তিনি হযরত আব্দুল্লাহ ইবনে আব্বাস (রা:)-কে উদ্ধৃত করেন, যিনি বলেন: “আমি আমার খালা মায়মুনা (রা:)-এর সাথে ছিলাম; এমন সময় রাসূলুল্লাহ (দ:) সেখানে আসেন এবং ওই হাদীসের বাকি অংশ ব্যক্ত করেন, যা গুনদার বর্ণনা করেছিলেন, আর নিঃসন্দেহে এই কথাও যোগ করেন, “আমাকে নূর (আলো) করুন।” খ। ইমাম মুসলিম (রহ:) এটি তাঁর সহীহ গ্রন্থের ‘সালাত আল-মুসাফিরীন’ অধ্যায়ে বর্ণনা করেন। ইমাম আহমদ (রহ:)-ও নিজ ’মুসনাদ’ কেতাবে শক্তিশালী সনদে এটি বর্ণনা করেন, তবে ওপরে উদ্ধৃত প্রথম রওয়ায়াতের বিপরীত দিক হতে; যার ফলে হুজূর (দ:)-এর ভাষ্য এ রকম হয়: “আর আমাকে নূর (আলো) করুন”, অথবা তিনি বলেছিলেন, “আমার জন্যে নূর সৃষ্টি করুন।” ইমাম ইবনে হাজর (রহ:) তাঁর ‘ফাতহুল বারী’ (১৯৮৯ ইং সংস্করণ, ১১:১৪২) কেতাবে ইবনে আবি আসিমের রচিত ‘কেতাব আল-দু’আ’-এর উদ্ধৃতি দেন যা’তে বিবৃত হয়েছে: “আর আমাকে মন্ঞ্জুর করুন নূরের ওপর নূর” (ওয়া হাবলী নূরান ‘আলা নূর)। মহানবী (দ:)-এর শরীর মোবারকের অন্যান্য অংশের কথা উল্লেখকারী এই হাদীসের আরও বহু নির্ভরযোগ্য বর্ণনা রয়েছে। ইমাম ইবনে হাজর বলেন যে ইমাম আবু বকর ইবনে আরবী (রহ:)-এর হিসেবমতে সমস্ত বর্ণনায় হুজূর পূর নূর (দ:)-এর নিজের জন্যে প্রার্থিত নূরের সংখ্যা ২৫টি। এগুলো নিম্নরূপ: মহানবী (দ:)-এর কলবে নূর জিহ্বায় নূর শ্রবণশক্তিতে নূর দৃষ্টিতে নূর ডানে, বামে, সামনে, পেছনে, ওপরে এবং নিচে নূর আত্মাতে নূর বক্ষে নূর পেশীতে নূর মাংসে নূর রক্তে নূর চুলে নূর চামড়ায় নূর হাড়ে নূর ”আমায় আলোর ওপর আলো দিন” ”আমায় আলো করুন”। রওযায় নূর ”আমার জন্যে আলো বৃদ্ধি করুন” ”আমায় অসীম আলো দিন” ৪। “নুরে মুহাম্মাদি” আল্লাহ্ সবার পূর্বে আনেক আগে সৃষ্টি করেছেন এবং সেই নূর থেকে জাবতিও সকল জিনিস সৃষ্টি করেহচেন। অনেকে এই সকল মতবাদ গুলিকে একত্র করে, নবী নুরের তৈরি এমন একটা নূর ভাবেন। আবার আনেকেই বলতে চান নবী কেবল ই মাটির তইরি। আবার অনেকে, নূর শব্দের বিভিন্ন ব্যাবহারিক অর্থ হিসাবে গ্রহন করেন। আমি এখানে ১,২ ও ৩ নং মতবাদ নিয়ে আলোচনা করব না, এই বিসয়ে যেমন ১। কেন আল্লাহ্ নবী কে নুর বল্লেন, ২। নবী কি নূর অথবা বাটির তৈরি না উভয়ের মিস্রন। ৩। নবী কি ধরনের নূর চাইতেন আল্লাহর কাছে। --- এই সকল বিসয়ে আমি আলোচনা এখানে করব না। এই সকল বিসয়ে আনেক কিতাব ও তাফসির আছে, সকলে দেখে নিতে পারেন – আমি এখানে গুরুত্ব দিতে চাই ৪ নং বিসয়ে আদিতে তৈরি “নুরে মুহাম্মাদি” এর ব্যাপারে, এটি একটি ভিন্ন বিসয়, এ বিসয়ে আনেক হাদিছ আছে, কিতাব আছে। আমি অল্প কয়েকটা হাদিছ ও কয়েক্তি কিতাবের উক্তি কাজী সাইফুদ্দীন হোসেন সাহেবের আনুবাদ থেকে ও ইন্টারনেট থেকে নিম্নে উল্লেখ করলাম। যেমন 1. ইমাম কাজী আয়ায (রহ:) তাঁর প্রণীত ‘শেফা শরীফ’ গ্রন্থে মহানবী (দ:)-এর সুউচ্চ বংশ পরিচয় ও তার শ্রেষ্ঠত্ব-বিষয়ক অধ্যায়ে বলেন: “হযরত ইবনে আব্বাস (রা:) বর্ণনা করেন যে আল্লাহতা’লা হযরত আদম (আ:)-কে সৃষ্টি করারও ২০০০ বছর আগে মহানবী (দ:)-এর রূহ মোবারক মহান প্রভুর হুযূরে (উপস্থিতিতে) নূরের আকৃতিতে অস্তিত্বশীল ছিলেন। ওই নূর খোদাতা’লার প্রশংসা ও বন্দনা করতেন, আর ফেরেশতাকুল ওই নূরের প্রশংসা করতেন। আল্লাহতা’লা যখন হযরত আদম (আ:)-কে সৃষ্টি করেন, তখন তিনি ওই নূরকে আদম (আ:)-এর পবিত্র কোমরের পেছনের দিকে বিচ্ছুরণ করেন।” 2. আলী ইবনে হুসাইন (রহ:) তাঁর পিতা ইমাম হসাইন (রা:) হতে, তিনি তাঁর পিতা হযরত আলী (ক:) হতে বর্ণনা করেন যে হুযূর পাক (দ:) এরশাদ ফরমান, ‘আল্লাহতা’লা হযরত আদম (আ:)-কে সৃষ্টি করার চৌদ্দ হাজার বছর আগে আমি ছিলাম মহান প্রভুর উপস্থিতিতে একটি নূর (আলো)’।” 3. বর্ণিত আছে যে, হযরত জাবের ইবনে আব্দিল্লাহ (রা:) হুযূর পাক (দ:)-এর কাছে আরয করেন: “এয়া রাসূলাল্লাহ (দ:)! আপনার জন্যে আমার পিতা-মাতা কুরবান হোন। আল্লাহ সবার আগে কী/কাকে সৃষ্টি করেছিলেন তা আমাদের বলুন।” মহানবী (দ:) এরশাদ ফরমান: ”ওহে জাবের! আল্লাহ সর্বপ্রথম তাঁর নূর হতে তোমাদের নবী (দ:)-এর নূর সৃষ্টি করেছিলেন, আর ওই নূর তাঁর কুদরতের মাঝে অবস্থান করেন ততোক্ষণ, যতোক্ষণ মহান প্রভু ইচ্ছা করেন; ওই সময়ে অস্তিত্ব না ছিল লওহের, না ছিল কলমের, না বেহেশতের, না দোযখের, না জাহান্নামের, না ফেরেশতার, না আসমানের, না জমিনের। আর যখন আল্লাহতা’লা তামাম মাখলুকাত সৃষ্টি করার ইচ্ছা করলেন, তখন তিনি ওই নূরকে চারভাগে বিভক্ত করলেন: প্রথমটি দ্বারা বানালেন কলম; দ্বিতীয়টি দ্বারা লওহ; তৃতীয়টি দ্বারা আরশ; এবং চতুর্থটি দ্বারা বাকি সব কিছু।” . হাদীসবেত্তা আবদূল হক দেহেলভী, এই বর্ণনা লিপিবদ্ধ করেন নিজ পারসিক ‘মাদারিজুন্ নবুওয়াত’ পুস্তকে এবং বলেন যে এটা সহীহ (বিশুদ্ধ)। কিতাবের মাধ্যমে উলামায়ে ইসলামের মাঝে এই বর্ণনার ব্যাখ্যা-বিশ্লেষণ বিভিন্ন রকমের। তাঁদের বক্তব্য নিম্নে দেয়া হলো: a. সাইয়েদ আবুল হাসান আহমদ ইবনে আবদিল্লাহ নিজ ‘আল-আনওয়ার ফী মওলিদ আন্ নবী মোহাম্মদ ‘আলাইহে আল-সালাত আল-সালাম’ কেতাবে হযরত আলী (ক:) থেকে নিম্নের হাদীসটি বর্ণনা করেন; নবী পাক (দ:) এরশাদ ফরমান: “আল্লাহ ছিলেন এবং তাঁর সাথে কেউ ছিল না; তিনি সর্বপ্রথম তাঁর মাহবুবের নূর সৃষ্টি করেন; এর ৪০০০ বছরের মধ্যে না সৃষ্টি করা হয়েছিল পানি, না আরশ, না কুরসী, না লওহ, না কলম, না বেহেশত, না দোযখ, না পর্দা, না মেঘমালা, না আদম, না হাওয়া।” b. আলেম আবদুল হাই লৌক্ষ্মভী স্বরচিত ‘আল-আসার আল-মারফু’আ ফী আল-আখবার আল-মওদু’আ’ গ্রন্থে এর উদ্ধৃতি দেন এবং বলেন, “নূরে মোহাম্মদীর শ্রেষ্ঠত্ব প্রতিষ্ঠিত হয়েছে আবদ আল-রাযযাক (রহ:)-এর বর্ণনায়, যা’তে এর পাশাপাশি রয়েছে সমগ্র সৃষ্টিকুলের ওপরে ওর সুস্পষ্ট অগ্রাধিকার।” c. মোহাম্মদ ইবনে আল-হাজ্জ আল-আবদারী নিজ ‘আল-মাদখাল’ কেতাবে (দারুল কিতাব আল-আরবী) আল-খতিব আবু আল-রাবি’ মুহাম্মদ ইবনে আল-লায়েস প্রণীত ‘শেফা আস্ সুদূর’ গ্রন্থ হতে এর উদ্ধৃতি দেন, যা’তে আল-লায়েস বলেন, “আল্লাহ সর্বপ্রথম যা সৃষ্টি করেন তা মহানবী (দ:)-এর নূর; আর ওই নূর অস্তিত্ব পেয়েই আল্লাহর প্রতি সেজদা করেন। আল্লাহ ওই নূরকে চার ভাগে বিভক্ত করেন এবং ওর প্রথম অংশ দ্বারা আরশ, দ্বিতীয়টি দ্বারা কলম, তৃতীয়টি দ্বারা লওহ এবং চতুর্থটি খণ্ডিত করে বাকি সৃষ্টি জগতকে অস্তিত্ব দেন। অতএব, আরশের নূর সৃষ্ট হয়েছে রাসূলে পাক (দ:)-এর নূর থেকে; কলমের নূরও তাঁর নূর থেকে; লওহের নূরও তাঁর নূর থেকে; দিনের আলো, জ্ঞানের আলো, সূর্য ও চাঁদের আলো, এবং দৃষ্টিশক্তি ও দূরদৃষ্টি সবই তাঁর নূর হতে সৃষ্টি করা হয়েছে।” d. ’আল মাওয়াহিব আল লাদুন্নিয়া’ গ্রন্থে লিপিবদ্ধ ইমাম কসতলানী (রহ:)-এর ভাষ্যানুযায়ী হযরত আবদ্ আল-রাযযাক, তাঁর রচিত ‘মুসান্নাফ’ কেতাবে ওপরের ঘটনাটি বর্ণনা করেন; ইমাম যুরকানী মালেকীও এটি বর্ণনা করেন নিজ ‘শরহে মাওয়াহিব’ পুস্তকে (মাতবা’আ আল-’আমিরা,।) হযরত ‘আবদ্ আল-রাযযাক (রহ:)-এর রওয়ায়াতের বিশ্বাসযোগ্যতার ব্যাপারে কোনো সন্দেহ-ই নেই। ইমাম বুখারী (রহ:) তাঁর কাছ থেকে ১২০টি এবং ইমাম মুসলিম (রহ:) ৪০০টি বর্ণনা গ্রহণ করেছেন। e. আহমদ আবেদীন শামী, তিনি ইমাম ইবনে হাজর হায়তামী মক্কী (রহ:)-এর কৃত ‘আন-নি’মাত আল-কুবরা ‘আলাল ’আলম ফী মওলিদে সাইয়্যেদে ওয়ালাদে আদম’ পুস্তকের ব্যাখ্যামূলক বইয়ে এই হাদীসটি দলিল হিসেবে পেশ করেন। ইমাম ইউসুফ নাবহানী (রহ:) তাঁর ‘জওয়াহির আল-বিহার’ কেতাবে এর উদ্ধৃতি দেন। f. এহসান এলাহী যাহির, তার ‘হাদিয়্যাত আল-মাহদী’ বইয়ে বলে: “আল্লাহ তাঁর সৃষ্টি আরম্ভ করেন ’আল-নূর আল-মোহাম্মদিয়া’ তথা মহানবী (দ:)-এর নূর দ্বারা; অতঃপর তিনি আরশ সৃষ্টি করেন পানির ওপর; এরপর বাতাস এবং একে একে ’নূন’, ’কলম’, লওহ এবং মস্তিষ্ক সৃষ্টি করেন। অতএব, মহানবী (দ:)-এর নূর আসমান ও জমিনে যা কিছু বিরাজমান তা সৃষ্টিতে মৌলিক উপাদান বলে সাব্যস্ত হয়.....আর হাদীসে আমাদের কাছে যা বিবৃত হয়েছে, তাতে (বোঝা যায়) আল্লাহতা’লা প্রথমে কলম সৃষ্টি করেন; আরও প্রথমে সৃষ্টি করেন মস্তিষ্ক; এর দ্বারা যা বোঝানো হয়েছে তা হলো আপেক্ষিক বা তুলনামূলক শ্রেষ্ঠত্ব।” g. ইমাম কসতলানী (রহ:)-এর ‘মাওয়াহিব’ থেকে পুরো হাদীসখানা উদ্ধৃত করেন ইসমাঈল ইবনে মুহাম্মদ আজলুনী নিজ ’কাশফ আল-খাফা’ গ্রন্থে (মাকতাবাত আল-গাযযালী,)। h. সাইয়্যেদ মাহমুদ আলুসী তাঁর কৃত ‘তাফসীরে রুহুল মাআনী’ কেতাবে বলেন, “সবার প্রতি বিশ্বনবী (দ:)-এর রহমত হওয়ার বিষয়টি সম্পৃক্ত রয়েছে এই বাস্তবতার সাথে যে, তিনি-ই সৃষ্টির প্রাক্ লগ্ন থেকে সমগ্র সৃষ্টিজগতের জন্যে ঐশী করুণাধারার মাধ্যম/মধ্যস্থতাকারী (ওয়াসিতাত্ আল-ফায়দ আল-এলাহী ‘আলাল মুমকিনাত্ ‘আলা হাসাব আল-কাওয়াবিল); আর এ কারণেই তাঁর নূর (জ্যোতি)-কে সর্বপ্রথমে সৃষ্টি করা হয়, যেমনিভাবে বর্ণিত হয়েছে হাদীসে, ‘ওহে জাবের, আল্লাহ সর্বপ্রথম তোমাদের নবী (দ:)-এর নূরকে সৃষ্টি করেন’; আরও এরশাদ হয়েছে, ’আল্লাহ দাতা, আমি বণ্টনকারী’ (আল-কাসেম)। সূফীবৃন্দ, আল্লাহ তাঁদের ভেদের রহস্যের পবিত্রতা দিন, এই অধ্যায় সম্পর্কে অনেক কিছূ বলেছেন।” আলুসী ‘রুহুল মাআনী পুস্তকের অন্য আরেকটি এবারতে হযরত জাবের (রা:)-এর হাদীসটি দলিল হিসেবে পেশ করেন। i. ইমাম যুরকানী মালেকী (রহ:)-এর ‘শরহে মাওয়াহিব’ (মাতবা’আ আল-আমিরা) এবং দিয়ারবকরীর ’তারিখ আল-খামিস’ বইগুলোর ভাষ্যানুযায়ী ইমাম বায়হাকী ভিন্ন শব্দচয়ন দ্বারা এটি বর্ণনা করেন নিজ ‘দালাইল আন-নবুওয়া’ গ্রন্থে। শায়খ আবদুল কাদের জিলানী তাঁর ‘সিররুল আসরার ফী মা ইয়াহতাজু ইলাইহ আল-আবরার’ কেতাবে এর উদ্ধৃতি দেন। আলী ইবনে বুরহান আল-দ্বীন আল-হালাবী নিজ ‘সীরাহ’ (মাকতাবা ইসলামিয়্যা) পুস্তকে এ হাদীস দলিল হিসেবে পেশ করেন এবং এরপর বলেন, “(সৃষ্টিতে) যা কিছু অস্তিত্বশীল, মহানবী (দ:) যে সবার মূল এ হাদীস তাই প্রমাণ করে; আর আল্লাহ-ই সবচেয়ে ভাল জানেন।” j. ইসমাইল হাক্কী তাঁর ‘তাফসীরে রুহুল বয়ান’ শীর্ষক কেতাবে এই হাদীসকে দলিল হিসেবে পেশ করেন এবং বলেন: “জেনে রাখুন, ওহে জ্ঞানী-গুণীজন, আল্লাহ সর্বপ্রথম যে বস্তু সৃষ্টি করেন, তা আপনাদের মহানবী (দ:)-এর নূর.........আর তিনিই হলেন সকল সৃষ্টির অস্তিত্বশীল হবার কারণ এবং তাদের সবার প্রতি আল্লাহতা’লার পক্ষ থেকে করুণা........আর তিনি না হলে ওপরের ও নিচের জগতসমূহ সৃষ্টি করা হতো না।” ইমাম ইউসুফ নাবহানী এই হাদীসের উদ্ধৃতি দেন নিজ ‘জওয়াহির আল-বিহার’ গ্রন্থে , ইমাম ইবনে হাজর হায়তামী স্বরচিত ‘ফাতাওয়ায়ে হাদীসিয়্যা’ পুস্তকে বলেন যে হযরত আব্দুর রাযযাক এ হাদীস বর্ণনা করেছেন; তিনি এটি মহানবী (দ:)-এর মওলিদ-বিষয়ক নিজ কাব্যগ্রন্থ ‘আন-নি’মাতুল কুবরা’-এর ৩য় পৃষ্ঠায় উদ্ধৃত করেন। এখন উপরের উল্লেখিত দলিল দস্তাবেজ থেকে বুজা যাচ্ছে, যে ১৪০০ বৎসর পূর্বে আমাদের প্রিয় নবী (স) এই বিশ্ব ব্রহ্মাণ্ডের সৃষ্টি ও তার পরিবর্তন ব্যাপারে একটু ধারনা প্রকাশ করছেন। এবং বলছেন পাথমে আল্লাহ্ সৃষ্টি করেছেন একটা নূর বা লাইট বা আলো যা হচ্ছে প্রশংসিত, যা থেকে সকল বস্তুর সৃষ্টি অর্থাৎ “নুরে মুহাম্মাদি” । তবে এই হাদিস গুলির মুল প্রতিপাদ্য বিসয় হচ্ছে এই মাহা বিশ্ব ব্রহ্মাণ্ডের সৃষ্টি ও তার পরিবর্তন এর ধাপ বর্ণনা করা, হাদিস গুলিতে খুব একটা বিস্তারিত নাই, তার একটা কারন আছে তাহল নবী (স) অতি সুক্ষ ও গোপন গিয়ান, গপনে গপনে বিতরন করতেন, এই গিয়ান গুলি হাদিসে ঈসারা পাওয়া যায় কিন্তু বিস্তারিত পাওয়া যায় না। এই সকল গিয়ান সিখতে হলে বায়াতে তরিকতের মাধ্যমে তালিম গুলি নিতে হয়। এই বিশ্ব ব্রহ্মাণ্ডের সৃষ্টির ধাপ এর তালিম গুলি বুজতে সহজ করারা জন্য আমি নিম্নে বিজ্ঞ্যান এর আবিস্কার গুলি তুলে ধরলাম। এই আবিস্কার গুলি বুজতে পারলে “ নুরে মুহাম্মাদি” বুজতে সহজ হয়। বিজ্ঞ্যানিরা ও আবিস্কারে পেয়েছেন যে সৃষ্টির প্রাথমে একটা আলোর বা লাইট এর সৃষ্টি থেকে হয়েছিল, ইহাকে “ছি-এম-বি” বলা হয়, এই আবিস্কার টা হয়ে ছিলে ১৯৬০ এর দিকে, এই “ছি-এম-বি” আবিস্কার এর ফলে “কস্মলজিকাল” বিজ্ঞ্যানে একটা বড় ধরনের ধাপ উম্মচিত হয়েছিল, ইহার ফলে “বিগ বেং” থিউরি কে বিস্তারিত প্রামান করা সম্ভব হয়েছিল অর্থাৎ এই বিশ্ব ব্রহ্মাণ্ডের সৃষ্টির ধাপ কে বিস্তারিত বর্ণনা করা স্মভব হয়েছে। কাজেই “নুরে মুহাম্মাদি” সহ এই বিশ্ব ব্রহ্মাণ্ডের সৃষ্টির ধাপ বুজার জন্য আমি নিম্নে ইন্টারনেট থাকে “ছি-এম-বি” বিসয়ে কিছু লিখা তুলে ধরলাম। The Cosmic Microwave Background ________________________________________ Cosmology is the study of the beginning and evolution of the universe. In cosmology, cosmic microwave background (CMB) radiation (also CMBR, CBR, MBR, and relic radiation) is thermal radiation filling the observable universealmost uniformly. With a traditional optical telescope, the space between stars and galaxies (the background) is completely dark. However, a sufficiently sensitive radio telescope shows a faint background glow, almost exactly the same in all directions, that is not associated with any star, galaxy, or other object. This glow is strongest in the microwaveregion of the radio spectrum. The CMBs serendipitous discovery in 1964 by American radio astronomers Arno Penzias and Robert Wilson was the culmination of work initiated in the 1940s, and earned them the 1978 Nobel Prize. Cosmic background radiation is well explained as radiation left over from an early stage in the development of the universe, and its discovery is considered a landmark test of the Big Bang model of the universe. When the universe was young, before the formation of stars and planets, it was denser, much hotter, and filled with a uniform glow from a white-hot fog of hydrogen plasma. As the universe expanded, both the plasma and the radiation filling it grew cooler. When the universe cooled enough, protons and electrons combined to form neutral atoms. These atoms could no longer absorb the thermal radiation, and so the universe became transparent instead of being an opaque fog. Cosmologists refer to the time period when neutral atoms first formed as the recombination epoch, and the event shortly afterwards when photons started to travel freely through space rather than constantly being scattered by electrons and protons in plasma is referred to as photon decoupling. The photons that existed at the time of photon decoupling have been propagating ever since, though growing fainter and less energetic, since the expansion of spacecauses their wavelength to increase over time (and wavelength is inversely proportional to energy according to Plancks relation). This is the source of the alternative term relic radiation. The surface of last scattering refers to the set of points in space at the right distance from us so that we are now receiving photons originally emitted from those points at the time of photon decoupling. What, another acronym? CMB stands for Cosmic Microwave Background. It is also sometimes called the CBR, for Cosmic Background Radiation, although this is really a more general term that includes other cosmological backgrounds, eg infra-red, radio, x-ray, gravity-wave, neutrino. The CMB contains hugely more energy than any other cosmic radiation source, however, so it is the dominant component of the overall CBR spectrum. Other acronyms, such as CMBR, are also sometimes used! Whats Cosmic about it? We refer to it as cosmic because the only known source of this radiation is the early universe. It can now be firmly concluded that the CMB is the cooled remnant of the hot Big Bang itself. Why Microwave? Light comes in a range of wavelengths, from the shortest wavelength gamma-rays to the longest wavelength radio waves, with common-or-garden visible light in the middle. All of these signals are manifestations of the same underlying physical phenomenon, travelling packets of oscillating electric and magnetic fields, called electro-magnetic radiation. All of the forms of electromagnetic radiation travel at the same speed, the speed of light, which is 300,000 km/s. E-m radiation of different wavelengths will interact with matter in different ways. For example, radio waves are picked up by a radio receiver, your eye detects visible light, infra-red radiation warms your skin, x-rays penetrate your body, gamma-rays can give you radiation damage. Microwaves are the name given to radiation between the infra-red and radio region, with wavelengths typically in the 1mm to 10cm range. Some specific wavelengths of microwaves can be used to excite the molecules in foodstuff, so that you can use them to cook. It turns out that if you had a sensitive microwave telescope in your house you would detect a faint signal leaking out of your microwave oven, and from various other man-made sources, but also a faint signal coming from all directions that you pointed. This is the Cosmic Microwave Background. Why is it called a Background? We refer to this radiation as a background because we see it no matter where we look. It clearly doesnt come from any nearby objects, such as stars or clouds within our Galaxy, or even from external galaxies. It is clearly a distant, background source of radiation. You can think of the whole Universe as being filled with this background of microwave photons. How do I pronounce anisotropy? If youve never come across this word before, then (obviously) its new to you, and so even professional cosmologists sometimes pronounce it wrongly. This then is a good question, but hard to answer in plain text! Basically, the stress is on the third syllable, and the common mistake is to stress the fourth. The confusion presumably arises from knowing how to pronounce anisotropic, and then thinking that you just pronounce it the same way, but without the final consonant. Why does the CMB support the Big Bang picture? The basic point is that the spectrum of the CMB is remarkably close to the theoretical spectrum of what is known as a blackbody, which means an object in thermal equilibrium. Thermal equilibrium means that the object has had long enough to settle down to its natural state. Your average piece of hot, glowing coal, for example, is not in very good thermal equlibrium, and a blackbody spectrum is only a crude approximation for the spectrum of glowing embers. But it turns out that the early Universe was in very good thermal equilibrium (basically because the timescale for settling down was very much shorter than the expansion timescale for the Universe). And hence radiation from those very early times should have a spectrum very close to that of a blackbody. The observed CMB spectrum is in fact better than the best blackbody spectrum we can make in a laboratory! So it is very hard to imagine that the CMB comes from emission from any normal stuff (since if you try to make stuff at some temperature, it will tend to either emit or absorb preferentially at particular wavelengths). The only plausible explanation for having this uniform radiation, with such a precise blackbody spectrum, is for it to come from the whole Universe at a time when it was much hotter and denser than it is now. Hence the CMB spectrum is essentially incontrovertible evidence that the Universe experienced a hot Big Bang stage (thats not to say that we understand the initial instant, just that we know the Universe used to be very hot and dense and has been expanding ever since). In full, the three cornerstones of the Big Bang model are: (1) the blackbody nature of the CMB spectrum; (2) redshifting of distant galaxies (indicating approximately uniform expansion); and (3) the observed abundances of light elements (in particular helium and heavy hydrogen), indicating that they were cooked throughout the Universe at early times. Because of these three basic facts, all of which have strengthened over the decades since they were discovered, and several supporting pieces of evidence found in the last deacade or two, the Big Bang model has become the standard picture for the evolution of our Universe. Can I see the CMB for myself? In fact you can! If you tune your TV set between channels, a few percent of the snow that you see on your screen is noise caused by the background of microwaves. How come we can tell what motion we have with respect to the CMB? Doesnt this mean theres an absolute frame of reference? The theory of special relativity is based on the principle that there are no preferred reference frames. In other words, the whole of Einsteins theory rests on the assumption that physics works the same irrespective of what speed and direction you have. So the fact that there is a frame of reference in which there is no motion through the CMB would appear to violate special relativity! However, the crucial assumption of Einsteins theory is not that there are no special frames, but that there are no special frames where the laws of physics are different. There clearly is a frame where the CMB is at rest, and so this is, in some sense, the rest frame of the Universe. But for doing any physics experiment, any other frame is as good as this one. So the only difference is that in the CMB rest frame you measure no velocity with respect to the CMB photons, but that does not imply any fundamental difference in the laws of physics. What sort of telescope is used to observe the CMB? Like light at any other wavelength the general system is a dish to collect and focus the radiation, a way of feeding the radiation to the instruments, and then the instruments themselves which are used to detect and record the signals. For microwaves the dish, or set of dishes, is made of a material (metal) which reflects microwaves. The focussed radiation is transported to the receivers by means of wave-guides, which are pipes specially tuned to transmit microwave signals. Then the detectors come in two types. Bolometers involve technology developed to detect infra-red radiation. They are essentially tiny pieces of special materials which absorb the microwave radiation. This in turn induces a minute change in temperature which is detected by a thermal sensor. These temperature variations are picked up in an electrical circuit and stored on computer. The other technology involves high performance transistors, which work in much the same way as the input circuitry of a radio receiver, only very much more efficient at picking up microwaves. Again the signal is then picked up and stored electronically. If you are interested in more detail you might want to check out a nice concise text like Detection of Light from the Ultraviolet to the Submillimeter, by G.H. Rieke, Cambridge Press, 1996. Where did the photons actually come from? A very good question. We believe that the very early Universe was very hot and dense. At an early enough time it was so hot, ie there was so much energy around, that pairs of particles and anti-particles were continually being created and annihilated again. This annihilation makes pure energy, which means particles of light - photons. As the Universe expanded and the temperature fell the particles and anti-particles (quarks and the like) annihilated each other for the last time, and the energies were low enough that they couldnt be recreated again. For some reason (that still isnt well understood) the early Universe had about one part in a billion more particles than anti-particles. So when all the anti-particles had annihilated all the particles, that left about a billion photons for every particle of matter. And thats the way the Universe is today! So the photons that we observe in the cosmic microwave background were created in the first minute or so of the history of the Universe. Subsequently they cooled along with the expansion of the Universe, and eventually they can be observed today with a temperature of about 2.73 Kelvin. The cosmic microwave background (CMB) radiation is an emission of uniform, black body thermal energy coming from all parts of the sky. The radiation is isotropic to roughly one part in 100,000: the root mean square variations are only 18 µK,[7] after subtracting out a dipole anisotropy from the Doppler shift of the background radiation. The latter is caused by the peculiar velocity of the Earth relative to the comoving cosmic rest frame as the planet moves at some 371 km/s towards the constellation Leo.[citation needed] In the Big Bang model for the formation of the universe, Inflationary Cosmology predicts that after about 10−37 seconds[8] the nascent universe underwentexponential growth that smoothed out nearly all inhomogeneities. The remaining inhomogeneities were caused by quantum fluctuations in the inflaton field that caused the inflation event.[9] After 10−6 seconds, the early universe was made up of a hot, interacting plasma of photons, electrons, and baryons. As the universeexpanded, adiabatic cooling caused the plasma to lose energy until it became favorable for electrons to combine with protons, forming hydrogen atoms. Thisrecombination event happened when the temperature was around 3000 K or when the universe was approximately 379,000 years old.[10] At this point, the photons no longer interacted with the now electrically neutral atoms and began to travel freely through space, resulting in the decoupling of matter and radiation.[11] The color temperature of the decoupled photons has continued to diminish ever since; now down to 2.7260 ± 0.0013 K,[3] their temperature will continue to drop as the universe expands. According to the Big Bang model, the radiation from the sky we measure today comes from a spherical surface called the surface of last scattering. This represents the set of locations in space at which the decoupling event is estimated to have occurred[12] and at a point in time such that the photons from that distance have just reached observers. Most of the radiation energy in the universe is in the cosmic microwave background,[13] making up a fraction of roughly 6×10−5 of the total density of the universe.[14] Two of the greatest successes of the Big Bang theory are its prediction of the almost perfect black body spectrum and its detailed prediction of the anisotropies in the cosmic microwave background. The CMB spectrum has become the most precisely measured black body spectrum in nature.[6] The big bang It is now generally agreed among both astronomers and physicists alike that the Universe was created some 10 to 20 billion years ago in a leviathan explosion dubbed the Big Bang. The exact nature of the initial event is still cause for much speculation, and its fair to say that we know little if anything about the first instant of creation. Nevertheless we do know that the Universe used to be incredibly hotter and more dense than it is today. Expansion and cooling after this cataclysm of the Big Bang, resulted in the production of all of the physical contents of the Universe which we see today. Namely: light in the form of photons; matter in the form of leptons (electrons, positrons, muons) and baryons (protons, antiprotons, neutrons, antineutrons); more esoteric particles like neutrinos and perhaps some exotic dark matter particles; and the subsequent formulation of the Universes first chemical elements. The concept of the Big Bang was not immediately obvious to astrophysicists, but rather grew out of a steady accumulation of evidencegathered from both theoretical and observational research throughout the course of the 20th century. A wide range of theories attempting to explain the origin of the Universe were eventually discredited and superseded by the Big Bang hypothesis based upon the following critical considerations: • the current expansion, or Hubble flow, of the Universe. • the observed helium and deuterium abundances. • the cosmic background radiation. • the cosmological solutions of Einsteins equations. • agreement between various independent estimates of the age of the Universe. The Cosmic Microwave Background Radiation Perhaps the most conclusive (and certainly among the most carefully examined) piece of evidence for the Big Bang is the existence of an isotropic radiation bath that permeates the entire Universe known as the cosmic microwave background (CMB). The word isotropic means the same in all directions; the degree of anisotropy of the CMB is about one part in a thousand. In 1965, two young radio astronomers, Arno Penzias and Robert Wilson, almost accidentally discovered the CMB using a small, well-calibrated horn antenna. It was soon determined that the radiation was diffuse, emanated unifromly from all directions in the sky, and had a temperature of approximately 2.7 Kelvin (ie 2.7 degrees above absolute zero). Initially, they could find no satisfactory explanation for their observations, and considered the possibility that their signal may have been due to some undetermined systematic noise. They even considered the possibility that it was due to a white dielectric substance (ie pigeon droppings) in their horn! However, it soon came to their attention through Robert Dicke and Jim Peebles of Princeton that this background radiation had in fact been predicted years earlier by George Gamow as a relic of the evolution of the early Universe. This background of microwaves was in fact the cooled remnant of the primeval fireball - an echo of the Big Bang. If the universe was once very hot and dense, the photons and baryons would have formed a plasma, ie a gas of ionized matter coupled to the radiation through the constant scattering of photons off ions and electrons. As the universe expanded and cooled there came a point when the radiation (photons) decoupled from the matter - this happened about a few hundred thousand years after the Big Bang. That radiation cooled and is now at 2.7 Kelvin. The fact that the spectrum (see figure) of the radiation is almost exactly that of a black body (a physicists way of describing a perfect radiator) implies that it could not have had its origin through any prosaic means. This has led to the death of the steady state theory for example. In fact the CMB spectrum is a black body to better than 1% accuracy over more than a factor of 1000 in wavelength. This is a much more accurate black body than any we can make in the laboratory! By the early 1970s it became clear that the CMB sky is hotter in one direction and cooler in the opposite direction, with the temperature difference being a few mK (or about 0.1% of the overall temperature). The pattern of this temperature variation on the sky is known as a dipole, and is exactly what is expected if we are moving through the background radiation at high speed in the direction of the hot part. The inference is that our entire local group of galaxies is moving in a particular direction at about 600 km/s. In the direction we are moving the wavelengths of the radiation are squashed together (a blue-shift), making the sky appear hotter there, while in the opposite direction the wavelengths are stretched out (redshift), making the sky appear colder there. When this dipole pattern, due to our motion, is removed, the CMB sky appears incredibly isotropic. Further investigations, including more recent ones by the COBE satellite (eg Smoot et. al.), confirmed the virtual isotropy of the CMB to better than one part in ten-thousand. A map of the sky at microwave frequencies, showing that the CMB is almost completely the same in all directions. Given this level of isotropy, together with the accurate black-body spectrum, any attempt to interpret the origin of the CMB as due to present astrophysical phenomena (i.e. stars, dust, radio galaxies, etc.) is no longer credible. Therefore, the only satisfactory explanation for the existence of the CMB lies in the physics of the early Universe. The Cosmological Dark Ages The age of the universe is around 10 to 20 billion years. The early Universe was so hot and dense that it was like the conditions within a particle accelerator or nuclear reactor. As the Universe expanded it cooled, so that the average energy of its constituent particles decreased with time. All of the high energy particle and nuclear physics was over in the first 3 minutes (see the book of that name, written by Steven Weinberg in 1977). By that time all of the main constituents of the Universe had formed, including the light elements and the radiation. It is generally believed that little of note happened for the next 300,000 years or so. This period is sometimes referred to as the Dark Ages of the Universe. One way to learn about physical processes which might have occurred at these times is to search for minor deviations from a black-body in the spectrum of the CMB. An injection of energy, through for example a decaying exotic particle, could distort the spectrum a little away from the characteristic blackbody shape. So far no such distortions have been found, so we have no reason to believe that anything particularly exciting happened during this time. The important thing which happened at about 300,000 years after the Big Bang is that the Universe became cool enough for the atoms to become neutral. Before that time all of the protons and electrons existed as free ions moving around in a plasma. Every time that a proton snatched an electron it would be zapped by a photon with high enough energy to rip them apart again. Only after about a few hundred thousand years was the average temperature low enough that the protons could hold onto their electrons to form neutral hydrogen atoms. This period is referred to as the epoch of recombination (in general when atoms become neutral after being ionized we talk of them recombining -- here in fact the ions and electrons are combining for the first time, so it should perhaps be called combination!). When the Universe was ionized, the matter was constantly interacting with the radiation, ie photons were continually being scattered by ions and electrons. Looking back at the CMB we see the surface of last scattering, when the photons last significantly interacted with the matter. At earlier times the universe is opaque, and so we dont see back further than the epoch of recombination. Between last scattering and today the universe is almost totally transparent. So when we look at the CMB we are seeing, in each direction, out to when the radiation last scattered. This means we are effectively seeing back in time to a few hundred thousand years after the Big Bang. After the Universe recombined, the stars, galaxies and clusters of galaxies started to form. We know little in detail about this process, largely because it is a very complex physical process. One of the biggest uncertainties is understanding the seeds from which the galaxies and other structures grew. Everything that we see with optical telescopes (or telescopes in any other wavelength range) tells us about objects which have existed in the last 10 billion years or so. It becomes more and more difficult to probe conditions in the Universe at earlier times. Detailed observations of the CMB provide exactly the sort of information required to attack most of the major cosmological puzzles of our day. By looking for small ripples in the temperature of the microwave sky we can learn about the seed fluctuations as they existed 300,000 years after the Big Bang, and well before galaxies had started to form. We can also learn what the Universe as a whole was like back then: whether it was open or closed; what the dominant form of dark matter is; and how the Universe has been expanding since that time. Through careful examination of the Cosmic Microwave Background we can probe the cosmological Dark Ages. Temperature Fluctuation While the CMB is predicted to be very smooth, the lack of features cannot be perfect. At some level one expects to see irregularities, or anisotropies, in the temperature of the radiation. These temperature fluctuations are the imprints of very small irregularities which through time have grown to become the galaxies and clusters of galaxies which we see today. Measurements of cosmic background radiation Precise measurements of cosmic background radiation are critical to cosmology, since any proposed model of the universe must explain this radiation. The CMBR has a thermal black body spectrum at a temperature of 2.72548±0.00057 K,.[3] The spectral density peaks in the microwave range of frequencies. However spectral density can be defined either as (a) dEν/dν (as in Plancks law) or as (b) dEλ/dλ (as in Wiens displacement law), where Eν is the total energy at all frequencies up to and including ν, and Eλ is the total energy at all frequencies up to and including λ. On definition (a), the peak spectral density occurs at a frequency of 160.2 GHz, corresponding to a 1.873 mm wavelength. Using definition (b), the peak is at a wavelength of 1.06 mm, corresponding to a frequency of 283 GHz. The glow is very nearly uniform in all directions, but the tiny residual variations show a very specific pattern, the same as that expected of a fairly uniformly distributed hot gas that has expanded to the current size of the universe. In particular, the spatial variation in spectral density (the derivative of the spectral density function with respect to the angle of observation in the sky) contains small anisotropies, or irregularities, which vary with the size of the region examined. They have been measured in detail, and match what would be expected if small thermal variations, generated by quantum fluctuations of matter in a very tiny space, had expanded to the size of the observable universe we see today. This is a very active field of study, with scientists seeking both better data (for example, the Planck spacecraft) and better interpretations of the initial conditions of expansion. Although many different processes might produce the general form of a black body spectrum, no model other than the Big Bang has yet explained the fluctuations. As a result, most cosmologists consider the Big Bang model of the universe to be the best explanation for the CMBR. On December 20, 2012, the Nine-year WMAP data and related images were released.[4][5] Graph of cosmic microwave background spectrum measured by the FIRAS instrument on theCOBE, the most-precisely measured black bodyspectrum in nature,[6] theerror bars are too small to be seen even in enlarged image, and it is impossible to distinguish the observed data from the theoretical curve HISTORY and Advance Discussion The cosmic microwave background was first predicted in 1948 byRalph Alpher, and Robert Herman.[26][27][28] Alpher and Herman were able to estimate the temperature of the cosmic microwave background to be 5 K, though two years later they re-estimated it at 28 K. This high estimate was due to a mis-estimate of the Hubble constant by Alfred Behr, which could not be replicated and was later abandoned for the earlier estimate. Although there were several previous estimates of the temperature of space, these suffered from two flaws. First, they were measurements of the effectivetemperature of space and did not suggest that space was filled with a thermal Planck spectrum. Next, they depend on our being at a special spot at the edge of the Milky Way galaxy and they did not suggest the radiation is isotropic. The estimates would yield very different predictions if Earth happened to be located elsewhere in the Universe.[29] The 1948 results of Alpher and Herman were discussed in many physics settings through about 1955, when both left the Applied Physics Laboratory at Johns Hopkins University. The mainstream astronomical community, however, was not intrigued at the time by cosmology. Alpher and Hermans prediction was rediscovered byYakov Zeldovich in the early 1960s, and independently predicted byRobert Dicke at the same time. The first published recognition of the CMB radiation as a detectable phenomenon appeared in a brief paper by Soviet astrophysicists A. G. Doroshkevich and Igor Novikov, in the spring of 1964.[30] In 1964, David Todd Wilkinson and Peter Roll, Dickes colleagues at Princeton University, began constructing a Dicke radiometer to measure the cosmic microwave background.[31] In 1965, Arno Penzias and Robert Woodrow Wilson at the Crawford Hilllocation of Bell Telephone Laboratories in nearby Holmdel Township, New Jersey had built a Dicke radiometer that they intended to use for radio astronomy and satellite communication experiments. Their instrument had an excess 3.5 K antenna temperature which they could not account for. After receiving a telephone call from Crawford Hill, Dicke famously quipped: Boys, weve been scooped.[1][32][33]A meeting between the Princeton and Crawford Hill groups determined that the antenna temperature was indeed due to the microwave background. Penzias and Wilson received the 1978 Nobel Prize in Physics for their discovery.[34] The interpretation of the cosmic microwave background was a controversial issue in the 1960s with some proponents of the steady state theory arguing that the microwave background was the result ofscattered starlight from distant galaxies.[35] Using this model, and based on the study of narrow absorption line features in the spectra of stars, the astronomer Andrew McKellar wrote in 1941: It can be calculated that the rotational temperature of interstellar space is 2 K.[15] However, during the 1970s the consensus was established that the cosmic microwave background is a remnant of the big bang. This was largely because new measurements at a range of frequencies showed that the spectrum was a thermal, black body spectrum, a result that the steady state model was unable to reproduce.[36] The Holmdel Horn Antenna on which Penzias and Wilson discovered the cosmic microwave background. Harrison, Peebles, Yu and Zeldovich realized that the early universe would have to have inhomogeneities at the level of 10−4 or 10−5.[37][38][39] Rashid Sunyaev later calculated the observable imprint that these inhomogeneities would have on the cosmic microwave background.[40] Increasingly stringent limits on the anisotropy of the cosmic microwave background were set by ground based experiments during the 1980s. RELIKT-1, a Soviet cosmic microwave background anisotropy experiment on board the Prognoz 9 satellite (launched 1 July 1983) gave upper limits on the large-scale anisotropy. The NASA COBEmission clearly confirmed the primary anisotropy with the Differential Microwave Radiometer instrument, publishing their findings in 1992.[41][42] The team received the Nobel Prize in physics for 2006 for this discovery. 9 year WMAP image of theCMB temperature anisotropy (2012).[4][5] Inspired by the COBE results, a series of ground and balloon-based experiments measured cosmic microwave background anisotropies on smaller angular scales over the next decade. The primary goal of these experiments was to measure the scale of the first acoustic peak, which COBE did not have sufficient resolution to resolve. This peak corresponds to large scale density variations in the early universe that are created by gravitational instabilities, resulting in acoustical oscillations in the plasma.[43] The first peak in the anisotropy was tentatively detected by the Toco experiment and the result was confirmed by theBOOMERanG and MAXIMA experiments. These measurements demonstrated that the geometry of the Universe is approximately flat, rather than curved.[47] They ruled out cosmic strings as a major component of cosmic structure formation and suggested cosmic inflation was the right theory of structure formation.[48] The second peak was tentatively detected by several experiments before being definitively detected by WMAP, which has also tentatively detected the third peak.[49] As of 2010, several experiments to improve measurements of the polarization and the microwave background on small angular scales are ongoing. These include DASI, WMAP, BOOMERanG, QUaD, Planck spacecraft, Atacama Cosmology Telescope, South Pole Telescope and the QUIET telescope. Relationship to the Big Bang This section may be too technical for most readers to understand. Please help improve this section to make it understandable to non-experts, without removing the technical details. The talk page may contain suggestions. (September 2011) The cosmic microwave background radiation and the cosmological redshift-distance relation are together regarded as the best available evidence for the Big Bang theory. Measurements of the CMB have made the inflationary Big Bang theory the Standard Model of Cosmology.[50] The discovery of the CMB in the mid-1960s curtailed interest in alternatives such as the steady state theory.[51] The CMB essentially confirms the Big Bang theory. In the late 1940s Alpher and Herman reasoned that if there was a big bang, the expansion of the Universe would have stretched and cooled the high-energy radiation of the very early Universe into the microwave region and down to a temperature of about 5 K. They were slightly off with their estimate, but they had exactly the right idea. They predicted the CMB. It took another 15 years for Penzias and Wilson to stumble into discovering that the microwave background was actually there. The CMB gives a snapshot of the universe when, according to standard cosmology, the temperature dropped enough to allow electrons and protons to form hydrogen atoms, thus making the universe transparent to radiation. When it originated some 380,000 years after the Big Bang—this time is generally known as the time of last scattering or the period of recombination or decoupling—the temperature of the universe was about 3000 K. This corresponds to an energy of about 0.25 eV, which is much less than the 13.6 eV ionization energy of hydrogen.[52] Since decoupling, the temperature of the background radiation has dropped by a factor of roughly 1,100[53] due to the expansion of the universe. As the universe expands, the CMB photons are redshifted, making the radiations temperature inversely proportional to a parameter called the universes scale length. The temperature Tr of the CMB as a function of redshift, z, can be shown to be proportional to the temperature of the CMB as observed in the present day (2.725 K or 0.235 meV):[54] Tr = 2.725(1 + z) For details about the reasoning that the radiation is evidence for the Big Bang, see Cosmic background radiation of the Big Bang. Primary anisotropy The power spectrum of the cosmic microwave background radiation temperature anisotropy in terms of the angular scale (or multipole moment). The data shown come from the WMAP (2006),Acbar (2004) Boomerang(2005), CBI (2004), and VSA(2004) instruments. Also shown is a theoretical model (solid line). The anisotropy of the cosmic microwave background is divided into two types: primary anisotropy, due to effects which occur at the last scattering surface and before; and secondary anisotropy, due to effects such as interactions of the background radiation with hot gas or gravitational potentials, which occur between the last scattering surface and the observer. The structure of the cosmic microwave background anisotropies is principally determined by two effects: acoustic oscillations and diffusion damping (also called collisionless damping or Silk damping). The acoustic oscillations arise because of a conflict in the photon–baryon plasma in the early universe. The pressure of the photons tends to erase anisotropies, whereas the gravitational attraction of the baryons—moving at speeds much slower than light—makes them tend to collapse to form dense haloes. These two effects compete to create acoustic oscillations which give the microwave background its characteristic peak structure. The peaks correspond, roughly, to resonances in which the photons decouple when a particular mode is at its peak amplitude. The peaks contain interesting physical signatures. The angular scale of the first peak determines the curvature of the universe (but not the topology of the universe). The next peak—ratio of the odd peaks to the even peaks—determines the reduced baryon density.[55] The third peak can be used to get information about the dark matter density.[56] The locations of the peaks also give important information about the nature of the primordial density perturbations. There are two fundamental types of density perturbations—called adiabatic andisocurvature. A general density perturbation is a mixture of both, and different theories that purport to explain the primordial density perturbation spectrum predict different mixtures. • Adiabatic density perturbations the fractional additional density of each type of particle (baryons, photons ...) is the same. That is, if at one place there is 1% more energy in baryons than average, then at that place there is also 1% more energy in photons (and 1% more energy in neutrinos) than average. Cosmic inflation predicts that the primordial perturbations are adiabatic. • Isocurvature density perturbations in each place the sum (over different types of particle) of the fractional additional densities is zero. That is, a perturbation where at some spot there is 1% more energy in baryons than average, 1% more energy in photons than average, and 2% less energy in neutrinos than average, would be a pure isocurvature perturbation. Cosmic strings would produce mostly isocurvature primordial perturbations. The CMB spectrum can distinguish between these two because these two types of perturbations produce different peak locations. Isocurvature density perturbations produce a series of peaks whose angular scales (l-values of the peaks) are roughly in the ratio 1:3:5:..., while adiabatic density perturbations produce peaks whose locations are in the ratio 1:2:3:...[57] Observations are consistent with the primordial density perturbations being entirely adiabatic, providing key support for inflation, and ruling out many models of structure formation involving, for example, cosmic strings. Collisionless damping is caused by two effects, when the treatment of the primordial plasma as fluid begins to break down: • the increasing mean free path of the photons as the primordial plasma becomes increasingly rarefied in an expanding universe • the finite depth of the last scattering surface (LSS), which causes the mean free path to increase rapidly during decoupling, even while some Compton scattering is still occurring. These effects contribute about equally to the suppression of anisotropies at small scales, and give rise to the characteristic exponential damping tail seen in the very small angular scale anisotropies. The depth of the LSS refers to the fact that the decoupling of the photons and baryons does not happen instantaneously, but instead requires an appreciable fraction of the age of the Universe up to that era. One method of quantifying how long this process took uses the photon visibility function (PVF). This function is defined so that, denoting the PVF by P(t), the probability that a CMB photon last scattered between time t and t+dt is given by P(t)dt. The maximum of the PVF (the time when it is most likely that a given CMB photon last scattered) is known quite precisely. The first-year WMAP results put the time at which P(t) is maximum as 372,000 years.[58]This is often taken as the time at which the CMB formed. However, to figure out how long it took the photons and baryons to decouple, we need a measure of the width of the PVF. The WMAP team finds that the PVF is greater than half of its maximum value (the full width at half maximum, or FWHM) over an interval of 115,000 years. By this measure, decoupling took place over roughly 115,000 years, and when it was complete, the universe was roughly 487,000 years old. Late time anisotropy Since the CMB came into existence, it has apparently been modified by several subsequent physical processes, which are collectively referred to as late-time anisotropy, or secondary anisotropy. When the CMB photons became free to travel unimpeded, ordinary matter in the universe was mostly in the form of neutral hydrogen and helium atoms. However, observations of galaxies today seem to indicate that most of the volume of the intergalactic medium (IGM) consists of ionized material (since there are few absorption lines due to hydrogen atoms). This implies a period of reionization during which some of the material of the universe was broken into hydrogen ions. The CMB photons are scattered by free charges such as electrons that are not bound in atoms. In an ionized universe, such charged particles have been liberated from neutral atoms by ionizing (ultraviolet) radiation. Today these free charges are at sufficiently low density in most of the volume of the Universe that they do not measurably affect the CMB. However, if the IGM was ionized at very early times when the universe was still denser, then there are two main effects on the CMB: 1. Small scale anisotropies are erased. (Just as when looking at an object through fog, details of the object appear fuzzy.) 2. The physics of how photons are scattered by free electrons (Thomson scattering) induces polarization anisotropies on large angular scales. This broad angle polarization is correlated with the broad angle temperature perturbation. Both of these effects have been observed by the WMAP spacecraft, providing evidence that the universe was ionized at very early times, at a redshift more than 17[clarification needed]. The detailed provenance of this early ionizing radiation is still a matter of scientific debate. It may have included starlight from the very first population of stars (population III stars), supernovae when these first stars reached the end of their lives, or the ionizing radiation produced by the accretion disks of massive black holes. The time following the emission of the cosmic microwave background—and before the observation of the first stars—is semi-humorously referred to by cosmologists as the dark age, and is a period which is under intense study by astronomers (See 21 centimeter radiation). Two other effects which occurred between reionization and our observations of the cosmic microwave background, and which appear to cause anisotropies, are the Sunyaev–Zeldovich effect, where a cloud of high-energy electrons scatters the radiation, transferring some of its energy to the CMB photons, and the Sachs–Wolfe effect, which causes photons from the Cosmic Microwave Background to be gravitationally redshifted or blueshifted due to changing gravitational fields. E polarization measurements as of March 2008 in terms of angular scale (or multipole moment). The polarization is much more poorly measured than the temperature anisotropy. Polarization Main article: Polarization in astronomy The cosmic microwave background is polarized at the level of a few microkelvin. There are two types of polarization, called E-modes and B-modes. This is in analogy to electrostatics, in which the electric field (E-field) has a vanishing curl and the magnetic field (B-field) has a vanishing divergence. The E-modes arise naturally from Thomson scattering in a heterogeneous plasma. The B-modes, which have not been measured and are thought to have an amplitude of at most 0.1 µK, are not produced from the plasma physics alone. They are a signal from cosmic inflation and are determined by the density of primordialgravitational waves. Detecting the B-modes will be extremely difficult, particularly as the degree of foreground contamination is unknown, and the weak gravitational lensing signal mixes the relatively strong E-mode signal with the B-mode signal.[59] Microwave background observations Main article: Cosmic microwave background experiments Subsequent to the discovery of the CMB, hundreds of cosmic microwave background experiments have been conducted to measure and characterize the signatures of the radiation. The most famous experiment is probably the NASA Cosmic Background Explorer (COBE) satellite that orbited in 1989–1996 and which detected and quantified the large scale anisotropies at the limit of its detection capabilities. Inspired by the initial COBE results of an extremely isotropic and homogeneous background, a series of ground- and balloon-based experiments quantified CMB anisotropies on smaller angular scales over the next decade. The primary goal of these experiments was to measure the angular scale of the first acoustic peak, for which COBE did not have sufficient resolution. These measurements were able to rule outcosmic strings as the leading theory of cosmic structure formation, and suggested cosmic inflation was the right theory. During the 1990s, the first peak was measured with increasing sensitivity and by 2000 theBOOMERanG experiment reported that the highest power fluctuations occur at scales of approximately one degree. Together with other cosmological data, these results implied that the geometry of the Universe is flat. A number of ground-based interferometers provided measurements of the fluctuations with higher accuracy over the next three years, including the Very Small Array, Degree Angular Scale Interferometer (DASI), and the Cosmic Background Imager (CBI). DASI made the first detection of the polarization of the CMB and the CBI provided the first E-mode polarization spectrum with compelling evidence that it is out of phase with the T-mode spectrum. In June 2001, NASA launched a second CMB space mission, WMAP, to make much more precise measurements of the large scale anisotropies over the full sky. WMAP used symmetric, rapid-multi-modulated scanning, rapid switching radiometers to minimize non-sky signal noise.[53] The first results from this mission, disclosed in 2003, were detailed measurements of the angular power spectrum at a scale of less than one degree, tightly constraining various cosmological parameters. The results are broadly consistent with those expected from cosmic inflation as well as various other competing theories, and are available in detail at NASAs data bank for Cosmic Microwave Background (CMB) (see links below). Although WMAP provided very accurate measurements of the large scale angular fluctuations in the CMB (structures about as broad in the sky as the moon), it did not have the angular resolution to measure the smaller scale fluctuations which had been observed by former ground-based interferometers. A third space mission, the ESA (European Space Agency) Planck Surveyor, was launched in May 2009 and is currently performing an even more detailed investigation. Planck employs both HEMT radiometers andbolometer technology and will measure the CMB at a smaller scale than WMAP. Its detectors were trialled in the Antarctic Viper telescope as ACBAR (Arcminute Cosmology Bolometer Array Receiver) experiment—which has produced the most precise measurements at small angular scales to date—and in the Archeops balloon telescope. Additional ground-based instruments such as the South Pole Telescope in Antarctica and the proposed Clover Project, Atacama Cosmology Telescope and the QUIET telescope in Chile will provide additional data not available from satellite observations, possibly including the B-mode polarization. Data reduction and analysis Raw CMBR data from the space vehicle (i.e. WMAP) contain foreground effects that completely obscure the fine-scale structure of the cosmic microwave background. The fine-scale structure is superimposed on the raw CMBR data but is too small to be seen at the scale of the raw data. The most prominent of the foreground effects is the dipole anisotropy caused by the Suns motion relative to the CMBR background. The dipole anisotropy and others due to Earths annual motion relative to the Sun and numerous microwave sources in the galactic plane and elsewhere must be subtracted out to reveal the extremely tiny variations characterizing the fine-scale structure of the CMBR background. The detailed analysis of CMBR data to produce maps, an angular power spectrum, and ultimately cosmological parameters is a complicated, computationally difficult problem. Although computing a power spectrum from a map is in principle a simple Fourier transform, decomposing the map of the sky into spherical harmonics, in practice it is hard to take the effects of noise and foreground sources into account. In particular, these foregrounds are dominated by galactic emissions such as Bremsstrahlung, synchrotron, and dust that emit in the microwave band; in practice, the galaxy has to be removed, resulting in a CMB map that is not a full-sky map. In addition, point sources like galaxies and clusters represent another source of foreground which must be removed so as not to distort the short scale structure of the CMB power spectrum. Constraints on many cosmological parameters can be obtained from their effects on the power spectrum, and results are often calculated using Markov Chain Monte Carlo sampling techniques. CMBR dipole anisotropy From the CMB data it is seen that our local group of galaxies (the galactic cluster that includes the Solar Systems Milky Way Galaxy) appears to be moving at 627±22 km/s relative to the reference frame of the CMB (also called the CMB rest frame, or the frame of reference in which there is no motion through the CMB) in the direction of galactic longitude l = 276±3°, b = 30±3°.[60] This motion results in an anisotropy of the data (CMB appearing slightly warmer in the direction of movement than in the opposite direction).[61] The standard interpretation of this temperature variation is a simple velocity red shift and blue shift due to motion relative to the CMB, but alternative cosmological models can explain some fraction of the observed dipole temperature distribution in the CMB.[62]
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