PHYM432 Relativity and Cosmology 20. Cosmology - Cosmic Microwave Background

Transcription

1 PHYM432 Relativity and Cosmology 20. Cosmology - Cosmic Microwave Background The Big bang theory is supported by three major observations 1) the expansion of the universe 1930s Edwin Hubble observed the redshift of distant galaxies 2) primordial nucleosynthesis Abundances of the elements can be predicted based on nuclear reaction rates and the temp/press of the early universe. Good agreement with observations. 3) the cosmic microwave background the surface of last scattering, where photons were finally free to propagate. A blackbody spectra redshifted to 3 degrees Kelvin.

2 primordial nucleosynthesis - atoms made as the universe cools. Initially comprised of mainly protons and neutrons, which then can combined to form the first light elements. Nuclear reaction rates are well known physics. These elements comprise the first stars. The elements made from those stars give us the abundances of the periodic table we see today. Convincing, as physics is well understood. all the atoms formed in the first 1000 seconds

3 Cosmic microwave background - After the first elements formed in the first few hours the universe was still hot and dense and the atoms were all ionized. Any photons endlessly Thompson-scattered of off all the electrons. Once the density drops far enough and temperatures become cool enough, electrons and the nuclei of the light elements present were free to combined to form the first atoms. This is the epoc of recombination. Once the density of free electrons was low enough, the photons were free to propagate though space. The epoch began once hydrogen could re-combined, ionization potential E=13.6 ev (T=150,000 K). As the number of photons vastly outnumbered the number of baryons (we re in the radiation dominated era remember), the gas remained ionised until the Universe had further cooled to 3004 K, and the photons were much less energetic and non-ionizing.

4 These leftover photons comprise a blackbody spectra, with a temperature from the surface of last scattering.

5 The blackbody spectra has been redshifted (z=1100) from expansion all the way from optical wavelengths, to microwave wavelengths where it is today. T r =2.728(1 + z) kelvin The microwave background is a near Perfect black body curve. The peak of the BB is at 2.7 degrees above absolute zero, which is the temperature the universe has dropped to after 13.7 billion years. discovered in 1965 by accident at Bell labs, Nobel prize given in 1978

6 CMB Anisotropies 0-4 kelvin ±3.352 mkelvin Due to Motion of the Galaxy proves we are not one of Weyl s privileged observers

7 CMB Anisotropies ±3.352 mkelvin ±18 μkelvin

8 CMB Anisotropies ±18 μkelvin no-galaxy The clumpness of the CMB image is due to fluctuations in temperature of the CMB photons. Changes in temperature are due to changes in density of the gas at the moment of recombination (higher densities equal higher temperatures). Since these photons are coming to us from the last scattering epoch, they represent fluctuations in density at that time.

9 CMB Anisotropies WMAP high res The origin of these fluctuations are primordial quantum fluctuations from the very earliest moments during inflation that are echo'ed in the CMB at recombination. Currently, people believe that these quantum fluctuations grew to greater than galaxy-size during the inflation epoch, and are the source of structure in the Universe. Without these fluctuations, gravity from matter would not have been pulled together to form stars and galaxies, and the homogenous & isotropic conditions would leave the Universe perfectly symmetric (Boring).

10 If we have small wrinkles or hills and valleys early on in the universe (the quantum fluctuations), matter will tend to fall into the valleys, eventually producing dense regions that become the sites of galaxies. simulation producing cosmic web dense

11 The blue bands are snapshots of the wrinkles in the density of the universe at various times. As time goes on, matter falls into these wrinkles and starts to build heavier and heavier objects. The crucial period when this process of gravitational attraction and infall can occur is related to an important concept in cosmology called the horizon. Like the horizon on the earth, it is the point beyond which we're unable to look. Heuristically, if there is a large clump in the universe we only know to fall toward it once it comes into the horizon.

12 When looking at the CMB across widely separated angles (like 180 degrees) we re looking on such large scales gravitational infall hasn t started yet. This is the signature of the primordial quantum fluctuations themselves.

13 On small scales (~1 degree across the sky) gravitational infall has started, and we re looking back at the structure formation at work. Matter is clumping together between the hills, together in the valleys, on their way to making superclusters of galaxies. What we re looking at on these scales is actually sound.

14 What we re looking at on these scales is actually sound.

15 The photons are behaving as a gas, just like the air. Sound waves in the air are just traveling compressions and rarefactions of the gas we hear. The most common example of a photon gas in equilibrium is Black Body Radiation. Photons in the early universe carry sound waves as gravity tries to compress the photon gas and pressure tries to resist it. The compressed photon gas becomes hotter, and the sound waves we see in the CMB are hot and cold spots on the sky. the resulting spectrum of sound waves are useful in determining the basic properties and evolution of the universe.

16 Observing the CMB today (oscillations frozen - dense & rarified regions), seeing angular variations of hot/cold regions in the sky. idealised 2D universe, with only linear fluctuations/dense regions at 1 scale.

17 Inflation potentially puts fluctuations at all scales, large and small.

18 The sound spectrum is characterised using spherical harmonics and their associated Legendre polynomials. These can be thought of as the spherical version of a Fourier transform Y lm (, ) =e im P m l (cos ) The peaks are the oscillating modes caught at their extrema during recombination. They form a harmonic series based on the distance sound can travel at recombination. The first peak compressed once. The second (smaller scales) compressed once then rarified... large scales small scales

19 The position of the 1st peak (and the whole spectrum) depends sensitively on the spatial Curvature of the Universe. Curvature acts as a lens, bending the light of the traveling CMB photons through the universe. In a positively curved universe, small objects appear larger, moving the peaks to smaller angles (higher l ) for smaller values of curvature. We know the actual size of the CMB spots, as we can calculate the sound speed and sound horizon, the standard rod (~0.27 Mpc). c s = c/ p b / r

20

21 (Credit: WMAP Science Team) WMAP has determined the CMB helping provide a standard model for cosmology. CDM A universe with dark energy and Cold Dark Matter The universe is 13.7 billion years old and spatially FLAT thus has very near the critical energy density. Currently 72% of the energy budget is dark energy, 23% dark matter, 4.6% atoms.

22 For much more see the excellent tutorials at:

23 Burning Questions? -how do you calculate the amount of dark matter/dark energy? - What happens when you fall into a black hole? - can we make a black hole -double relativity -can we travel in time -evidence for wormholes