The one misconception most people have regarding the Big Bang is the idea that the Big Bang originated from a singular point IN space – but that’s not the case! Instead, the entirety of our current 3D space WAS a singular point (this is difficult to picture, since we can’t really imagine a 3D-point without any 3D-space surrounding it, but that’s exactly what it was).
Therefore, the correct thing to say is that the Big Bang happened everywhere within the volume of our 3D space simultaneously (instead of saying the Big Bang happened from a single point, which suggests there was one point IN space where everything started). Yes, very difficult to picture mentally since there’s no real ‘outside-in’ view of that event that you could imagine visually, only an ‘inside-out’ view, since 3D space does not exist outside of that event.
So because the Big Bang happened everywhere IN space, we can look in any direction to see its remnants.
Currently we can ‘see’ (using special cameras since it’s not in the visible spectrum) back to a time when the universe was approximately 377’000 years young (compared to the total age of the universe, that’s pretty close to t=0).
The reason we can currently see only back to t≈377’000 years is that before this point in time, the universe was not yet transparent but instead it was opaque.
What does that mean the Universe was not yet transparent? While space was still rapidly expanding (though not quite as rapidly anymore as before), it was still a lot smaller than it is today and therefore a lot denser and hotter. Before t≈377’000 years, electrons had not yet coupled with protons due to this immense heat and pressure, but were bouncing around ‘freely’. That hot dense soup of plasma was also glowing, meaning photons were released constantly and everywhere, but since there were free electrons everywhere, these photons could travel only very short distances before colliding somewhere with an electron, causing them to be scattered in a different direction, so instead of traveling on long straight paths, photons were just scattering around (try to imagine the interior of our sun – it’s really bright but you can’t see shit because that plasma soup is not transparent and instead the photons that reach your retina are those that were released right in front of your eyes, that’s how the universe looked more or less when it was younger than 377’000 years).
Then, very suddenly at t≈377’000 years, the pressure and temperature had dropped enough (due to expansion of space) that all these free electrons combined with protons and were no longer unbound. This time-period is called “recombination”, and since photons now suddenly could travel freely without colliding with free electrons, the universe turned from an opaque soup to mostly transparent free-space.
That point in time at t≈377’000 years is called the “last scattering surface”. The term “surface” here does not refer to a 2D surface within 3D space but instead to a 3D volume in 4D space (think of the 3D ‘slice’ within 4D space-time at exactly t≈377’000 years, that’s the “last scattering surface”, spanning the entirety of 3D space during a single point in time).
Exactly those photons that were scattered by interaction with free electrons for the very last time at t≈377’000 years are flying through transparent space on straight paths ever since and are the very photons that make up the “Cosmic Microwave Background Radiation”, or ‘CMBR’. So, the CMBR was released simultaneously everywhere in space and in all possible directions at a single point in time and is just traveling through space more or less freely ever since. Thus, when we ‘see’ the CMBR (using these special cameras), the photons that are picked up by the camera have been flying through space ever since and are now for the very first time colliding with something again (in this case, they collided with the sensor of that special camera). Since photons travel at the speed of light, it means we are directly ‘seeing’ a spherical slice of the “last scattering surface”, a spherical region located around us with a radius exactly (age_of_the_universe – 377’000 years) * c ≈ 13.8 billion light years in comoving distance (which corresponds to about 40 billion light years in propper distance since the universe kept expanding since these photons were released).
So quite similarly to how we see an outside-in view of the outer most surface layer of the sun, called photosphere, at t≈(now – 8 minutes) when we look at the sun from earth, we see an inside-out view of the “cosmic photosphere”, meaning we directly see that hot dense plasma how it looked when it was only 377’000 years old (since the light was traveling for approximately 13.8 billion years and space continuously expanded while those photons were traveling through it they got “stretched out” however and have thus decreased in frequency and photon energy, we call this stretching “red shift”).
Now while 377’000 years is very very young on a cosmic time-scale, there was still a lot of stuff happening in that time which we cannot look at so literally yet as we can look at the last scattering surface (aka. the cosmic photosphere aka. the CMBR).
Once we improve our neutrino detection technology however, this will change. While the last scattering surface for photons was at t≈377’000 years, the neutrino equivalent of that event, called “neutrino decoupling”, happened at t≈1 second. Unfortunately it’s a lot more difficult to detect neutrinos than it is to detect photons, since neutrinos only interact gravitationally and via the weak force with matter, but we are nevertheless making progress in that regard and one day we can build a camera to take a picture of the “Cosmic Neutrino Background” CNB, the neutrino equivalent of the CMBR, which will mean that we will directly ‘see’ an inside-out view of a spherical sub-region of the universe when it was exactly 1 second old.
– xkrbl
