CMB

Cosmic Microwave Background Radiation

In 1965, Penzias and Wilson of Bell Laboratories,

made the startling discovery that an all pervasive background radiation fills the Universe. Since then several further missions, Cosmic Background Explorer (COBE), Wilkinson Microwave Anisotropy Probe (WMAP) and Planck, have probed deeper into the properties and meaning of the CMB.
The CMB has properties:

The spectrum of the CMB is well-described by a thermal spectrum (a blackbody spectrum) with the tiny temperature, T ~ 2.725 Kelvin or - 455 Farenheit (or - 270 C) !!!

The CMB is uniform and isotropic, i.e., it has very close to the same temperature all across the sky -- temperature differences of < 0.004 % on angular scales of 7 degrees excluding a well-known 0.12 % variation known as the dipole anisotropy found after subtracting the average temperature from the image and finer, lower amplitude temperature variations found after subtracting the average temperature and the dipole anisotropy from the image.

That the CMB is the same to within 0.004 % on scales < 7 degrees, excluding the dipole anisotropy, has severe implications for ideas upon the structure of the Universe was when it was young. We will address some of the questions in our discussion of the Cosmological Principle.
The dipole anisotropy arises due to the motion of the Milky Way galaxy through the Universe. The Local Group of galaxies which contains the Milky Way is moving in the direction of Hydra with a speed of around 500 kilometers per second. This causes the portion of the Universe toward which we are moving to be blueshifted and so to appear hotter than the region from which we are receding. See the top image to the right. This effect arises from the Doppler shift.
The low-amplitude, small-scale fluctuations seen in the CMB are both good and may turn out to be rather vexing at the same time. We address questions related to the small imperfections in the CMB later.

Why is the CMB Strong Support for the Big Bang Model?

Basic Idea: The CMB is the relic of the Big Bang. In the past when the Universe was small, it was much hotter. Then, as the Universe expanded, it cooled until today, the current temperature of the radiation is only 2.7 Kelvin!
More Detailed Idea: In the context of scientific models, the important point is that the Big Bang theory was adopted after Hubble showed that the Universe was expanding. Subequent theoretical investigations of the Big Bang model by George Gamow (in the 1940s) led to prediction of the CMB well-before the discovery of the CMB was made. Gamow found that not only was the Universe very hot at birth, but he was able to estimate how hot the Universe was when it was young. Gamow was thus able to make a prediction for how hot the Universe would be today if it had been expanding and cooling for 10 billion or so years. Gamow make a fairly precise prediction for the current temperature of the Universe. Gamow predicted a temperature on the order of 5-10 Kelvin, not 1,000 Kelvin, not 0 Kelvin, not 1 billion Kelvin. Gamow's prediction was verified with the discovery of Penzias and Wilson of the CMB in 1965. This result provides strong support for the Big Bang theory.
Gamow's prediction was slightly off because, for among other reasons, the Hubble constant had not yet been pinned down.

What are we looking at when we look at the CMB?

Well, thermal radiation (blackbody radiation) typically comes from objects that are hot and dense (and opaque), e.g., the coils on your electric stove, the wire filament in a light bulb, ... . When the Universe was young, it was hot and dense and opaque and so emitted a blackbody spectrum. This is why the CMB is so well-described by a blackbody spectrum.

So, when the Universe was young, it was opaque which prevented us from seeing anything beyond our noses; we saw only the nearby Universe.

As the Universe expanded, it became transparent to light during what is called the Epoch of Recombination. After the Epoch of Recombination the Universe was transparent to radiation and so after this, we can see essentially throughout the Universe.

When we look at the CMB, we see the Universe as it was at the time of Recombination, roughly 378,000 years after the Universe started its expansion. This means that when we look at the Universe using light, we cannot see to the beginning of the Universe, we only see to Recombination when the Universe had a temperature of around 3,000 Kelvin.

Today, after around another 13.7 billion years has passed (after Recombination), we see the Universe after it has expanded in size to nearly 1,000 times its size that it had at Recombination. This large expansion is why the Universe has cooled to 2.7 Kelvin.

WMAP and Planck beyond pinning down the properties of the CMB, sharpened and fine-tuned our current picture of the Universe, in particular, finding that the composition of the Universe is

68.5 % of the Universe is Dark Energy, the unknown material which causes the expansion of the Universe to speed-up!
the rest is mainly Matter, the material that slows the expansion rate of the Universe (via gravity). Of the 31.5 % that is matter, Dark Matter makes up around 26.6 % of the Universe and Normal Matter, the stuff that makes up planets, stars, and most importantly, us, makes up around 4.9 % of the Universe.
the rest of the Universe is made up of the CMB (light, photons).

In addition, Planck and WMAP found that the expansion rate of the Universe, as measured by the current value for the Hubble constant, is H_o = 67.4 km/sec per Mpc. Recall that the best measurement made by work following Hubble was H_o = 73 km/sec per Mpc.
The disagreement is real in that each observation is secure; the small difference in H_o does not appear to be attributable to experimental error. This disagreement is what is referred to as the Hubble Tension. To resolve the Hubble Tension might mean that there is a piece of physics missing from the current models or that a piece of the physics that goes into the current models needs modification. Either possibility is interesting.

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