1. The Expanding Universe
Notes p.52
The Expanding Universe
3. The Evidence
Additional reading: Higher Physics for CfE, p
In 1920’s Hubble’s work led to the idea that the Universe is expanding. In the years that followed, other pieces of evidence strengthened this theory. Even nowadays, further evidence continues to be revealed.

2. Evidence of Universe Expansion
3 important pieces of evidence for an expanding Universe (and the Big Bang Theory) are listed below.
Hubble’s observations of redshift tells us that most galaxies and stars are moving apart.
2. Temperature Measurements of Cosmic Microwave Background (CMB) Radiation are as predicted by the Big Bang Theory.
3. The darkness of the night sky tells us there must be galaxies whose light has still to reach us. So they can’t be infinitely old.

3. “Hubble’s observations of redshift tells us that most galaxies and stars are moving apart.”
As explained earlier in your notes, the redshift from stellar objects, particularly distant galaxies, allows measurements of their velocities and distances from us to be made.
So, going back in time, it would seem that all galaxies were once together at some fixed point: all matter and energy in the Universe was once concentrated into an infinitesimally small, infinitely hot, infinitely dense, something - a singularity!

4. “Temperature Measurements of Cosmic
“Temperature Measurements of Cosmic Microwave Background (CMB) Radiation are very cold, as predicted by the Big Bang Theory.”
If there was really a Big Bang, then photons of light emitted during it would have expanded in wavelength into the microwave region, by now, as the Universe expanded.
This radiation should be uniformly distributed across the Universe and have a “peak wavelength” (more on this later) indicating a very low temperature due to cooling of the Universe during its expansion.

5. In 1965, the idea of CMBR became a reality.
It was detected in deep space (where hotter radiation from stars and galaxies was not hiding it). These Cosmic Microwaves are believed to have been emitted DURING the birth of our Universe – during the “BIG BANG”.
It was accidentally detected by 2 scientists, Penzias and Wilson, while they were investigating the use of microwaves for Telecommunications. They even received the Nobel Prize for their accidental discovery!
This evidence is arguably the most significant evidence for The Big Bang Theory and Expanding Universe.

6. “The darkness of the night sky tells us there
“The darkness of the night sky tells us there must be galaxies whose light has still to reach us. So they can’t be infinitely old.”
More of an observation than a measurement, the question “Why is the Night Sky Dark” has been raised for centuries. (In 1576 by Digges; in 1610 by Kepler and formally in 1823 by Heinrich Olbers).
The question stemmed from the assumption at the time that the Universe was infinite and static.

7. If this was true then surely there would be an
If this was true then surely there would be an infinite wall of star light at night time, in all directions.
And yet there was so much darkness at night, between the stars.
BUT if the Universe is NOT infinitely old, and big, then either there might be no galaxies yet formed in some directions, or some that are so distant, their light has not yet reached us.

8. This would account for large areas of darkness
This would account for large areas of darkness between the nearer stars and galaxies whose light has reached us.
And, if the Universe is NOT infinitely old, then it was created at some time in the past, which is consistent with Big Bang Theory.

9. Dark Matter
By measuring the orbital speeds of stars in a galaxy, the mass of the galaxy can be estimated.
However, the rotational speeds at the outer parts of observed galaxies are only possible if the mass of the galaxy is much bigger than what we can detect. There seems to be mass that’s “missing”!
This “missing mass” is now called “Dark Matter”.

10. Dark Energy
It is expected that gravitational effects should slow down the expansion of the Universe.
HOWEVER, measurements indicate that the rate of expansion is in fact increasing!
So, there must be some energy we cannot detect that overcomes gravity.
We call it Dark Energy.

11. So 96% !!! of what’s out there, we know virtually nothing about!
We can only assume it’s there because of how it affects the stuff we can observe.