re: No matter what direction we point a telescope, we always look toward the Big Bang - why?

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No matter what direction we point a telescope, we always look toward the Big Bang

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I haven’t read the entire thread, but this description is taking some liberties. It’s more accurate to say that no matter what direction we point a telescope, we always see the cosmic microwave background radiation. The CMBR is the oldest electromagnetic radiation (e.g. light) that we can see. This is a somewhat important distinction. Let’s assume the Big Bang theory is correct for the sake of argument:

Immediately after the Big Bang, the universe expands rapidly. This is referred to as the period of inflation. The universe consists of a hot, dense plasma which is cooling as the universe expands. As the plasma cools, subatomic particles are combining to create larger and larger particles. First quarks and gluons, then hadrons, then protons and neutrons, then electrons, and then photons. The universe cools enough to form these particles over the first ~10 seconds.

At this point, the plasma contains a ton of photons but also a ton of free electrons. The interaction between the photons and free electrons/protons leads to an effect known as scattering, which effectively turns the plasma into a glowing fog.

For the next ~20 minutes, nuclear fusion occurs as neutrons and protons combine to create atomic nuclei. However, the overall substance remains a plasma and free electrons continue to scatter the light. The universe is effectively opaque.

Now the universe continues to cool while the rate of expansion slows down for the next ~370,000 years. Eventually the universe cools enough for the plasma to “recombine” into ordinary matter - although “recombination” is a bit of a misnomer since it was never combined in the first place. Recombination is basically the equivalent of condensation or freezing. The plasma’s temperature drops to the point where the free electrons and atomic nuclei combine to create hydrogen atoms. At this point the expansion rate slows down considerably.

This process happens quickly (relatively speaking) across the universe. Think about it like if you have a pot full of sugar dissolves in water and cool it until it becomes supersaturated. Once it starts crystallizing, the entire pot clouds up with sugar particles. Only in this case, the “opaque” universe actually becomes transparent because the free electrons are no longer blocking/scattering photons. Additionally, recombination itself releases photons as the hydrogen atoms are formed.

The photons released from recombination are what we know as the CMBR. Now let’s look at the special aspect of this. The picture below from Wikipedia gives an idea of how scientists think the universe expanded after the Big Bang:



The “afterglow pattern” is the CMBR. Basically, if you were at any point in space at the time of recombination, you would see a bright glow in all directions, originating from your immediate surroundings, as the plasma combined into hydrogen atoms.

After a year, the light from your immediate surroundings would have dissipated and the space would be transparent. However, the light from recombination that happened one light-year away would just be arriving.

100 years after that, you would be surrounded by 100 light-years of transparent space but you would still see the light (which I’ll just call CMBR from now on) that originated 100 light-years away.

Fast forward 200-400 million years. Now the matter in the universe has finally combined (due to gravity) and started to form plasma again - creating the first stars. So at some point you start seeing light from stars in all directions. At 600 million years, you are seeing light from stars that are <200 million (or so) light years away. You see nothing at all 200-570 million light years away. And at about 570 million light years away, you see the CMBR.

At no point can you see past the CMBR because the universe was opaque at that point, blocking any light beyond the CMBR from reaching you. So you can’t “see” the Big Bang.. but you see the glow from its aftermath in all directions.