Something that has long baffled me about the Big Bang. We all started out from that point then boom! Everything came into being rapidly and we started expanding, and we have been expanding for 14 billion years or so, correct? When we look into the sky using the Hubble space telescope we can see right up to the plasma that was there only a few hundred thousand of years after the big event, yes?

How? We have been travelling away from that point for billions of years, and at less than the speed of light, isn't that right? So the light from that event would have kept pace with us for sme of that time, and we would be seeing some kind of decay even if it's time sequence was slowed down by our regression, wouldn't we? So we shouldn't be able to see anything from 14 billion yers aago unless we ended up exactly where we are right now 14 billion years ago and the light has been travelling since then to get to us.
:headspinning:
I know I'm wrong about this, but can't get my head around WHY I'm wrong Now I now what it feels like to be a member of the fun boy three...

You should try asking in Brotoro's thread.

The Hubble space telescope can't see back to the point where the plasma cooled to the point of becoming transparent (since that light has been red-shifted down into the microwave region of the EM spectrum), but that's what we are detecting as the cosmic background radiation…so, yes, we can see back that far.

The photons that got freed at the era of recombination (which happened about 380,000 years after the big bang) are now reaching us from points in space that are about 13.8 billion light years away… But the matter that released those photons has been moving away from us because of the expansion of the universe during these 13+ gigayears, so that matter is now out at 46 to 47 billion light years from us. So even though the 'size of the observable universe' is a radius of ~46.5 billion light years, we can only see what that stuff looked like when it released the photons ~13.8 billion years ago.

TIL the term "gigayears".

Is it 13.4? I thought I'd read closer to 13.7 awhile ago.

This is why we can't have a space program.

Why, because our debt is expanding faster than space time?

Current best estimate of time since the big bang is closer to 13.8 billion years. But I made a mistake when I was subtracting the time of recombination from that (and got 13.4). It should be closer to 13.8 billion. Correction has been made.

Thanks.

Just to clarify: The matter that emitted the photons we now see in the cosmic background radiation were released ~13.8 billion years ago, so we are seeing what that stuff looked like 13.8 billion years ago (well, it actually looked like red hot gas, but those red light photons have been red-shifted down into microwaves). That material is now out at at ~46.5 billion light years. And although one might think that the matter must have been 13.8 billion light years from us when it released the photons, it was only about only about 42 million light years away at that time (all because of the expansion of the universe). So what LOOKS like stuff we are seeing at a distance of 13.8 billion light years was actually much closer than 13.8 billion light years when it emitted that light, and is now much further away than 13.8 billion light years.

Objects in telescope lens are closer than they appear.

How do we know all that?

I think my brain still can't quite cope with the fact that we still see the 14 billion years ago....I know it's my problem but I can't get past it lol...why are we seeing it at all? why after the time it has taken us to get to here, hasn't the light we see decayed because of time so that it is out of any perception that does not have a time machine to hand? Even if it's a small fraction of the actual time, why can we see so far back in space that w are looking so far back in time?

My. Brain. Hurts....

For once, if you tell me I'm being thick about this, I wont mind... I'm aware that others don't see this problem like I do, and so there is something I'm failing to grasp

You need mass to decay, and photons, as far as we know, are massless.

They might not be, but even in that case, they would last a long, long time. Also, there may be time dilation going on.

You take that back.

Well, I don't think we have confirmed if that much mass can decay. It might alter the fabric of spacetime as far as we know.

We can measure the expansion rate by looking at how fast galaxies move away from us versus their distance from us. The farther away a galaxy is, the faster it is receding from us. We see this when looking in all directions. This is what you would expect to see from any point in a uniformly expanding cosmos.

And when we look at the expansion velocity vs. distance at very far distances, we can see that the expansion rate has not been the same over all history, and the expansion rate is increasing now with time (although early on when everything was closer together, gravity would have been decreasing the expansion rate for a while).

This expansion tells us that everything must have been closer together in the past (obviously), but detailed models of this tell us how dense and hot the universe must have been at previous times. Therefore astronomers knew there should be a cosmic background radiation left over from when the universe was 380,000 years old (and got merely red-hot so that electrons could combine with nuclei to make atoms...which produces a transparent gas compared to a gas with all those ionized particles around).

We can detect the details of this background radiation (temperature, how lumpy it is, how it's polarized) that give us a very good picture of the conditions then (and our models must match this data).

If you "run the film" backward even further, the conditions of the universe were such that a certain fraction of hydrogen nuclei created from the Big Bang should have cooked up into helium at about three minutes after the bang (just by applying what we know about nuclear reaction rates to those conditions), and when we look at the primordial gas between galaxies now, we see the ratio of hydrogen and helium that is predicted by the theory.

So it all hangs together with the observations.

Many thanks, Brotoro.

Light is electromagnetic radiation: packets of energy that propagate through space according to wave equations. It does not get tired with time. What DOES happen is that as the space of the universe expands, the wavelength of those photons get stretched out. This is why the photons that got free from the red-hot gas that filled the universe earlier than 380,000 years after the Big Bang are now microwave photons...so they HAVE become imperceptible by the human eye... But we have radio receivers that can still detect them.

Also, what we are 'looking at' is the emissions from all of that gas that filled the universe...so there are a LOT of these microwave photons streaming through space. Back in the days when we had analog TVs connected to antennas, you could tune the TV to an empty channel and watch the static fill the screen. That static comes from various sources, but about 1% of that "snow" was caused by cosmic background radiation.

Just to see if I got it:

We measure how fast are the galaxies receding, and we come up with out Hubble constants and stuff like that.

Because of that, we know that, at one point, stuff was really close, and we know the conditions, so we can expect some microwave background radiation from that period.

We find it, and we build Plank maps and WMAPs and stuff like, which confirms it, and it turn, gives us even more knowledge about other stuff.

And since we know the was space expands, we know that the matter that released the radiation 16 billion years ago was some 42 million light years from us, and now it's over 46 billion years apart. That's because in the 16 billion years the universe expanded, and the photons took that much more time reaching us. And of course, in that period of time space kept expanding, and now the stuff is much farther away.