Cosmology

Cosmology is a broad term and is often qualified as follows:

• Physical cosmology - the application of science (literally physics) to the understanding of the Universe.
• Observational cosmology (cosmography) - scientific observations of the Universe.
• Religious cosmology - belief-based explanations of the Universe.
• Cosmogony - theories of how the Universe came to exist.

Hubble's law

• We have already covered Hubble's law in some depth.
• It marks the beginning of modern observational cosmology.
• It provided evidence that space-time was expanding, but, curiously, did not displace the Steady State theory of the Universe.
• The observations that finally did this came in 1964.

Horn Microwave Antenna - Holmdel, New Jersey

Source: NASA, Public Domain

Penzias and Wilson

• In the early 1960s, physicists Arno Penzias and Robert Wilson worked the Horn Antenna in Holmdel, built for communication research with satellites and radio astronomy.
• In testing the equipment, they encountered radio interference that they could not eliminate.
• The signal seemed to be uniform across the entire sky and didn't vary with time of day or year.
• This ruled out all terrestrial sources, and made solar system and even galactic sources seem unlikely.
• Two other explanations were considered: the whole Universe emitting in the microwaves, or a white dielectric deposit left by pigeons in the antenna.

The Cosmic Microwave Background - Discovery

• Only a few miles away at Princetown University, another team led by Robert H. Dicke were intending look for such a microwave background.
• They collaborated and soon established that it was indeed the whole Universe emitting in the microwave part of the spectrum.
• Penzias and Wilson were awarded the Nobel prize for physics for this discovery in 1978.
• The abbreviation CMB or CMBR (R for radiation) is often used.

CMB and the Big Bang models

• The CMB was predicted from Big Bang models that supposed the expanding Universe started in a hot, dense state.
• These models, though published, were not widely accepted because the Steady State theory was still predominant.
• The hot big bang models predicted that the early Universe would have emitted as a black body.
• When the Universe reached a temperature of 3000 K it became transparent to electromagnetic radiation and so no longer emitted as a black body.

CMB and redshift

• 3000 K corresponds to a spectrum that peaks near the red end of the visible spectrum.
• But the radiation of that time did not disappear - we now see it in the microwaves because it has been redshifted due to the expansion of space-time.
• It still seems like a black body, but one at 3 K.
• Note: it is sometimes said that the Universe cooled to 3 K. This is incorrect. We cannot meaningfully talk about it having temperature after the radiation "decoupled" from matter when the Universe became transparent.

CMB - Spectrum

Source: Quantum Doughnut, Public Domain

CMB data

• The graph on the previous page was taken by the famous COBE satellite.
• It shows that the data fits a black body spectrum with temperature 2.7260 ± 0.0013 K.
• The error bars on the graph are too small to be seen.
• No other object in nature shows such a clear fit to a black body spectrum.

Cosmic Background Explorer - COBE

• The COBE satellite was launched by NASA in 1989.
• Its FIRAS instrument obtained the CMB spectrum.
• Its DMR instrument looked for anisotropies (deviations from isotropy) of the CMB, and found them in temperature to be about 1 part in 100,000.
• This wasn't a surprise: some anisotropy must exist if we can hope to explain the current large scale structure of the Universe (but see the horizon problem later on).

COBE - temperature aniostropies

Source: NASA, Public Domain

WMAP

• NASA launched COBE's successor, the Wilkinson Microwave Anisotropy Probe in 2001.
• Its aim was to make more precise measurements of the temperature anisotropies that COBE had found.
• The goal was more profound that simply to improve precision - these measurements could greatly constrain cosmological models, telling us about:
  • the curvature of space-time in the Universe
  • the proportions of baryonic matter, dark matter and dark energy
  • the value of the cosmological models

WMAP - temperature anisotropies

Source: NASA, Public Domain

WMAP - 9 year results

• WMAP updated its results every two years until the end of the mission after 9 years.
• It established the following:
  • Hubble's constant H₀= 70.0±2.2 km s^-1 Mpc^-1
  • The curvature parameter is -0.037±0.04 (0 means flat space-time).
  • 72.1% of the Universe is composed of dark energy.
  • 23.3% is dark matter.
  • 4.6% is baryonic matter
  • The Universe is 13.74±0.11 billion years old.

Planck

• The Planck spacecraft was launched by the European Space Agency (ESA) in 2009.
• It aims to further refine the work of WMAP.
• In March 2013 its research team reported:
  • Hubble's constant H₀= 67.80±0.77 km s^-1 Mpc^-1
  • 68.3% of the Universe is composed of dark energy.
  • 26.8% is dark matter.
  • 4.9% is baryonic matter
  • The Universe is 13.798±0.037 billion years old.

Non CMB work

• The CMB results are amongst the most significant in observational cosmology in recent decades, but there are many other efforts.
• Observations of high redshift objects can be used to probe whether physical constants have varied over the history of the Universe
• Specifically, there is tantalising evidence from quasars that the fine structure constant α might have varied by one part in a million.
• Type Ia Supernovae first indicated a non-zero cosmological constant Λ and the existence of dark energy.
• Type Ia supernovae also allow us to construct high redshift Hubble Diagrams that can constrain cosmological models.

Hubble diagram from supernovas

Source: Copyright Ned Wright UCLA

Models and assumptions

• The results stated from the COBE, WMAP and Planck are based on the Lambda Cold Dark Matter or Λ-CDM cosmological models.
• These contain assumptions that, if found suspect, could radically alter the numerical results for the age of the Universe and compositions of matter and energy.
• The ± uncertainties stated earlier are statistical given the limitations of the instruments and data reduction, they do not account for the possible incorrectness of model assumptons.
• Alternative models are actively considered, but Λ-CDM is currently the most favoured.
• A number of aspects still await satisfactory explanation, most obviously, what is dark matter and dark energy?