Lecture 29: Observed Properties of the Universe

• Early astronomers Edmund Halley and Heinrich Olbers (1758 - 1840) wondered why the sky is dark at night.
• They wondered, if the sky is filled with an infinite number of stars, the sky shouldn't be dark.
• If you look in any direction, your line of sight should always hit a star, and no matter how far away it is, some of its light should hit your eyes. This statement holds for either stars or galaxies.
• This statement is known as Olbers' paradox.
• (Although the brightness of a star is inversely proportional to the square of the distance, the number of stars counted increases as the square of the distance.)

• Edgar Allen Poe (1809 - 1849) came up with the following solution to Olbers' paradox:
  
  Since light has a finite speed, if the Universe is not infinitely old, then light from the most distant sources has not had enough time to travel to us.
  
  
• Poe's solution suggests that the Universe had an early period when no stars (or galaxies) existed, and that they only formed some time ago. In other words the properties of the Universe are not constant in time.

• When we look at galaxies with large redshifts, we see that their properties tend to be different than galaxies with small redshifts.

• For instance, quasars only appear at larger redshifts.

• Galaxies in the Hubble Deep an Ultra-Deep Fields are much smaller than galaxies today.

• This suggests that the properties of galaxies have evolved with time and as an extension, the properties of the universe have changed with time.

• This suggests that the universe is not static or unchanging.

• Maps of the universe (Cfa, Los-Campanos, 2dF, SDSS and other redshift surveys) at distance scales of a few 100 Mpc show large superclusters (The Great Wall) and large voids with no galaxies.
• Maps of the universe at larger scales (out to z = 0.3) show that the distribution of galaxies is smoother when one averages over 1000 Mpc.
• At the largest distance scales that we can measure out to, there is no direction in the sky with more galaxies than the other directions.
• Since the distribution of galaxies at the largest scales is the same in all directions, we call the universe isotropic.
• This isotropy could be due to the Milky Way being at the "centre" of the universe.
• However most astronomers adopt the Copernican philosophy which states that we are not at a special location in the universe.
• If we are not at a special location, then observers living in another galaxy cluster will measure a similar sky map.
• This leads to the assumption that the universe is homogeneous in space.

5. Cosmic Microwave Background Radiation

• When we say that the sky looks dark at night, this is only because our eyes are sensitive to only a tiny part of the electromagnetic spectrum.
• If we look at any part of the sky with a radio telescope, we see a glow of radiation coming from all directions.
• The spectrum of the radiation from the sky can be measured and corresponds to blackbody radiation of a temperature of 2.73 K (Nobel Prize, 2006).
• (In order to see this spectrum we have to look at a part of the sky where the Milky Way and other nearby stars are absent.)
• This spectrum peaks a radio wavelength near 1mm which is sometimes called microwave.
• This radiation from the sky is called the Cosmic Microwave Background Radiation (CMBR).

• Hot emission ~5K was predicted by theorists as a leftover of the hot early stage of the Universe evolution.
• This 2.73 K emission was discovered by Robert Wilson and Arno Penzias in the early 1960's.
• They were testing a very sensitive microwave receiver which was to be used to communicate with a satellite.
• They were discouraged by a constant hiss which they continued to detect whenever they pointed the receiver at apparently empty parts of the sky.
• They soon learned that the "Big Bang" model of the universe predicts that there was an early phase of the universe's life when it was filled with radiation.
• The early predictions were that the radiation should have a temperature (measured today) less than 10 K.
• Penzias and Wilson were awarded the Nobel Prize in physics for their discovery.

Detailed CMBR Maps reveal fluctuations of temperature

• Modern measurements of the CMBR show that the temperature is very smooth, that it is almost constant in all directions.
• This smoothness lends support for the claim that the universe is isotropic (and homogeneous) at large distance scales .
• However, small temperature fluctuations should exist: some areas of the sky are hotter or cooler than 2.73 K by 3 x 10^-5 K
• Such fluctuations at the level 3 x 10^-5 K were discovered by the COBE satellite in 1992. (Nobel prize 2006)
• Now they are measured very accurately, for example by WMAP and recently launched Planck satellites.

• The Milky Way galaxy lies along the horizontal axis, and its light has been subtracted out of WMAP picture.

• The red spots are hotter than average.

• The blue spots are cooler than average.

• Cooler regions correspond to regions of the early universe which were denser than average.