Olbers paradox Why is the sky dark at night? Of course, the Sun’s gone down! But more careful consideration of this simple fact led early astronomers to get the first constraints on cosmological models

Olbers paradox Why… If the universe were infinite in size & contained an infinite number of stars that live forever, then every line-of-sight would eventually lead to a star Stars light dims as 1/r 2 but the volume of space sampled increases by the same factor So, the night sky should be as bright everywhere as the average surface of a star

Olbers paradox How can it be dark then? Pondered as early as Kepler and Newton. Newton wanted the universe to be infinite to avoid collapse under his theory of gravity One of the scientists associated with discussing the puzzle was Heinrich Olbers, and his name remained associated with “Olbers paradox” Some people suggested the distant stars light to be absorbed and thus diminished before reaching us No good - why?

Olbers paradox Any material which absorbed the starlight should heat up and re-emit it, we would see this gas glowing ! Flaw in arguments was assumption stellar lifetimes are infinite In fact, if we look far enough we look back to a time when no stars existed Further, an universe with finite age or which is expanding has a limit to how far we can see, light has to have had time to reach us

Olbers paradox Anyway, number of stars is too small, and stellar lifetimes to short to fill space with light The darkness of the night sky rules out the simplest idea that the universe is infinite and filled with unchanging stars

The Cosmic Microwave Background (CMB) However, there is more radiation filling the universe than that from stars Something called the Cosmic Background Radiation (CBR) fills the sky in all directions at wavelengths too long for our eyes to see

The Cosmic Microwave Background (CMB) The expanding universe does tell us something about this new version of Olbers paradox The expansion has caused CBR to redshift to longer wavelengths from its original energy! The cosmos must have once been ablaze with this radiation-this is the radiation predicted to have been produced in the Big Bang!

The Cosmic Microwave Background (CMB) Observational discovery of the CMB The Big Bang model What can we learn from the CMB?

1964 Penzias & Wilson (Bell-Labs) and antenna THE OBSERVATIONAL DISCOVERY OF THE COSMIC MICROWAVE BACKGROUND

CBR Arno Penzias & Robert Wilson (1964) –Attempted to study radio emission from our Galaxy using sensitive antenna built at Bell-Labs –Needed to characterize and eliminate all sources of noise –They never could get rid of a certain noise source… noise had a characteristic temperature of about 3 K –They figured out that the noise was coming from the sky, and was approximately the same in all directions…

THE HOT BIG BANG MODEL Penzias & Wilson had discovered radiation left over from the early universe… The big bang model… –Independently developed by James Peebles and George Gamov –They suggested that the universe started off in an extremely hot state –As the universe expands, the energy within the universe is spread over in increasing volume of space… –Thus the Universe cools down as it expands

CBR Why did they suggest this model? –If the early universe was hot (full of energy), a lot of features of the current universe could be explained… –Could explain where the matter that we see around us came from (H, He, Li created by the big bang) –Could explain the observed ratio of elements (nucleosynthesis occurred within first few minutes) –Predicted that there should be left over radiation in the present universe…

A brief look at the stages of the Universe’s life… We will discuss this diagram in detail in future classes. Most crude description… –t=0: The Big Bang –For first 380,000ys, universe is an expanding “soup” of tightly coupled radiation & matter –After 380,000yrs, radiation & matter “decouple”. Radiation field reduced enough (due to expansion) that protons can now capture electrons & hang on to them –Left over radiation that we see known as the CMB…

THE COSMIC BACKGROUND EXPLORER (COBE)

COBE The COBE mission –Built by NASA-Goddard Space Flight Center –Launched Nov –Purpose was to survey infra-red & microwave emission across the whole sky (BB predicts 3 K black body) –Primary purpose – to characterize the CMB Had a number of instruments on it: –FIRAS (Fair infra-red absolute spectrophotometer) –DMR (Differential Microwave Radiometer) –DIRBE (Diffuse Infrared background Experiment)

The CMB: DMR map of the microwave sky…

COBE Results Map of the microwave sky (frequency of 50GHz)… –We’re looking at the CMB –The map is very uniform. –Means that the CMB is extremely isotropic (i.e. the same in every direction we look) –Supports the idea that the universe is isotropic (one of the basic cosmological principles). –In fact, if we measure the universe to be isotropic, and we’re not located at a special place in the Universe, we can also deduce that the Universe is homogeneous!

The spectrum of the CMB (FIRAS)

CBR-Spectrum Spectrum has precisely the shape predicted by the theory… –So-called “Blackbody” spectrum –Characteristic temperature of 2.728K

Subtract off average level…

CBR What causes this pattern of redshift and blueshift?

CBR What causes this pattern of redshift and blueshift? Earths motion through space causes this dipole (anisotropy with 2 well- defined/opposite points) (Earth orbits Sun at 30 km/s, Sun orbits MW at 220 km/s, MW has motion around center of local group etc)

Subtract the “dipole”…

CBR Subtract off the dipole resulting from the Earth’s motion… Are left with… –Bright ridge corresponding to microwave emission from our Galaxy –Pattern of random fluctuations in the CMB

Subtract off the emission from our Galaxy…

CBR fluctuations Can use the different spectrum of the Galaxy’s emission and the CMB to distinguish them. So, can subtract off the emission from our Galaxy… Left with a random pattern of fluctuations in the CMB… correspond to temperature differences of 30 millionths of a Kelvin

CBR fluctuations What are these fluctuations… –The early universe was very close to being perfectly homogeneous –But, there were small deviations from homogeneity… some regions were a tiny bit colder and some were a tiny bit hotter. –When matter and radiation decoupled, this pattern of fluctuations was frozen into the radiation field. –We see this nowadays as fluctuations in the CMB.

CBR Fluctuations Why are the fluctuations important? –Before decoupling, fluctuations in the radiation field also meant fluctuations in the mass density –After decoupling, these small fluctuations in density can get amplified (slightly dense regions get denser and denser due to gravity). –These growing fluctuations eventually collapse to give galaxies and galaxy clusters. –So, by studying these fluctuations, we are looking at the “seeds” that grow to become galaxies, stars, planets…

Microwave Anisotropy Probe (MAP)

NASA mission to map out the fluctuations in the CMB in fine detail… –Will characterize these seeds for structure formation –Will determine fine detail of the CMB fluctuations that depend upon the curvature of space (k) and . Launched last year…

New WMAP results - Tues Feb 11 NASA's Wilkinson Microwave Anisotropy Probe (WMAP) Patterns in the big bang afterglow were frozen in place only 380,000 years after the big bang, a number nailed down by this latest observation. These patterns are tiny temperature differences within this extraordinarily evenly dispersed microwave light bathing the universe, which now averages a frigid 2.73 degrees above absolute zero temperature. WMAP resolves slight temperature fluctuations, which vary by only millionths of a degree. Theories about the evolution of the universe make specific predictions about the extent of these temperature patterns A NASA satellite has captured the sharpest-ever picture of the afterglow of the big bang Dates universe to 13.7 billion years.