E4.1 Olber’s paradox Why is the night sky dark? or Why isnt the night sky as uniformly bright as the surface of the Sun?Newton’s model of the universe assumed that the universewas infinite (in space and time) uniform and static.If the Universe has infinitely many stars, then it should beuniformly bright in all directions.
E4.1 Olber’s paradox If the Universe is eternal andinfinite and if it has an infinitenumber of stars, then the night skyshould be bright. Very distant stars contribute withvery little light to an observer onEarth but there are many of them.So if there is an infinite number ofstars, each one emitting a certainamount of light, the total energyreceived must be infinite, makingthe night sky infinitely bright, whichit is not.
E4.2 Olber’s paradox If we consider the Universe finite and expanding, theradiation received will be small and finite mainly for 2 reasons: There is a finite number of stars and each has a finitelifetime (they don’t radiate forever) and Because of the finite age of the Universe, stars that are faraway have not yet had time for their light to reach us. Also, The Universe is expanding, so distant stars are red-shiftedinto obscurity (contain less energy).
E4.3 Doppler effect In astronomy, the Doppler effect was originally studied in thevisible part of the electromagnetic spectrum. Today, theDoppler shift, as it is also known, applies to electromagneticwaves in all portions of the spectrum. Also, because of the inverserelationship between frequencyand wavelength, we candescribe the Doppler shift interms of wavelength. Radiationis redshifted when itswavelength increases, and isblueshifted when its wavelengthdecreases.
E4.3 Doppler effect In astronomy, the Doppler effect was originally studied inthe visible part of the electromagnetic spectrum. Today, theDoppler shift, as it is also known, applies to electromagneticwaves in all portions of the spectrum. Astronomersuse Doppler shiftsto calculateprecisely how faststars and otherastronomicalobjects movetoward or awayfrom Earth.
E4.4 Doppler effect Why is Doppler effect so important? In 1920’s Edwin Hubble and Milton Humanson realisedthat the spectra of distant galaxies showed a redshift, whichmeans that they are moving away from Earth. So, if galaxiesare moving away from each other then it they may havebeen much closer together in the past Matter was concentrated in one point and some “explosion” may have thrown the matter apart.
E4.5 Background radiation In 1960 two physicists, Dicke and Peebles, realising thatthere was more He than it could be produced by stars,proposed that in the beginning of the Universe it was at asufficiently high temperature to produce He by fusion. In this process a great amount of highly energetic radiationwas produced. However, as the Universe expanded andcooled, the energy of that radiation decreased as well(wavelength increased). It was predicted that the actualphotons would have an maximum λ corresponding to a blackbody spectrum of 3K. So, we would be looking for microwave radiation.
E4.5 Background radiationShortly after thisprediction, Penzias andWilson were working witha microwave aerial andfound that no matter inwhat direction theypointed the aerial itpicked up a steady,continuous backgroundradiation.
E4.5 Background radiation In every direction, there is a very low energy and veryuniform radiation that we see filling the Universe. This is calledthe 3 Degree Kelvin Background Radiation, or the CosmicBackground Radiation, or the Microwave Background. These names come aboutbecause this radiation isessentially a black body withtemperature slightly lessthan 3 degrees Kelvin(about 2.76 K), which peaksin the microwave portion ofthe spectrum.
E4.6 Background radiation Why is the background radiation an evidence for the Big Bang? The cosmic background radiation (sometimes called theCBR), is the afterglow of the big bang, cooled to a faintwhisper in the microwave spectrum by the expansion of theUniverse for 15 billion years (which causes the radiationoriginally produced in the big bang to redshift to longerwavelengths).
E4.6 Big Bang The Big Bang Model is a broadly accepted theory for theorigin and evolution of our universe. It postulates that 12 to 14 billion years ago, the portion ofthe universe we can see today was only a few millimetresacross. It has since expanded from this hot dense state into thevast and much cooler cosmos we currently inhabit. We can see remnants of this hot dense matter as the nowvery cold cosmic microwave background radiation which stillpervades the universe and is visible to microwave detectors asa uniform glow across the entire sky.
E4.6 Big Bang The singular point at which space, time, matter and energy were created. The Universe has been expanding ever since.Main evidence: Expansion of the Universe – the Universe is expanding(redshift) it was once smaller it must have startedexpanding sometime “explosion” Background radiation evidence of an hot Universe thatcooled as it expanded He abundance He produced by stars is little there isno other explanation for the abundance of He in the Universethan the Big Bang model.
E4.8 Fate of the Universe Universe Closed Open FlatEnough matter Not enough matter Critical density density is not enough to density is such that Universe will onlyallow an infinite gravity is too weak start to contractexpansion gravity will to stop the Universe after an infinitestop the Universe expanding forever amount of timeexpansion and cause itto contract (Big Crunch)
E4.9 Critical densityThe density of the Universe that separates a universe thatwill expand forever (open universe) and one that will re-colapse (closed universe).A universe with a density equal to the critical density iscalled flat and it will expand forever at a slowing rate. So, how do we measure the density of the Universe?
E4.9 Critical density If we take in account all the matter (stars) that we can seethen the total mass would not be enough to keep the galaxiesorbiting about a cluster centre. So, there must be some matter that can not be seen – darkmatter. This dark matter cannot be seen because it is too coldto irradiate. According to the present theories dark matter consists inMACHO’s and WIMPS
E4.11 Massive compact halo objects – brown and MACHO’s black dwarfs or similar cold objects and even black holes. Non-barionic weakly interacting massive WIMP’s particles (neutrinos among other particles predicted by physics of elementary particles)It seems that there is also what is called “dark energy”…