# When was Big Bang visible?

1. Oct 13, 2015

### Helios

The Big Bang is not visible to us because it is beyond the cosmological horizon. Yet long ago, I suppose it was visible. So how old was the Universe when observers got their last chance to see the Big Bang? Is this age called anything? Is it special?

2. Oct 13, 2015

### bapowell

The big bang exists in our past; it is not "outside our cosmological horizon". Remember, the big bang happened everywhere at once. We are still receiving its relic radiation in the form of the CMB.

3. Oct 13, 2015

### phinds

Google "Surface of Last Scattering" (the CMB always has been visible since that time, and always will be, although it will in the far distant future fade beyond detectability)

4. Oct 13, 2015

### Helios

I'm not talking about the CMB. Is there an age of the Universe that's too old to see? Isn't this what is beyond the cosmological horizon?

5. Oct 13, 2015

### phinds

You are talking about the CMB without realizing it. The CMB IS the place where it is not so much "too old" to see but "too dense in energetic particles" to see. There's some belief that with improved detectors we might someday be able to see some beyond that with neutrino detectors and/or gravity wave detectors, but as for now, we cannot get any information directly from beyond the CMB. There's a lot of good inference about what had to be happening, thought. See, for example, Steven Weinberg's "The First Three Minutes".

6. Oct 13, 2015

### bapowell

If light could have traveled freely since the Big Bang, then this light observed today originated from points on today's particle horizon. Beyond the particle horizon is more light from the Big Bang that has yet to reach us.

We mention the CMB because the universe was not transparent to light early on, and so we cannot "see" photons from distances beyond the last scattering surface. The CMB is the oldest light observable.

7. Oct 13, 2015

### Helios

I am not talking about the CMB and I realize that I am not talking about the CMB. I am talking about an idealized scenario where light could travel freely since the Big Bang, as bapowell describes. What I gather about the expanding universe is that it brings into effect light that we will never see. If light could travel freely since the Big Bang, is true or false that we will never be able to see it?

8. Oct 13, 2015

### Orodruin

Staff Emeritus
Then your question makes no sense. There will be places outside of our cosmological horizon, yes, but the Big Bang is not a place. It ocurred everywhere.

9. Oct 13, 2015

### Helios

If the Big Bang just emitted one photon, could I possibly see it? ( assuming I could see that one photon )

10. Oct 13, 2015

### Orodruin

Staff Emeritus
Again, the Big Bang is not a single place so your question makes no sense. If there was not a dense state of matter blocking your view you would see some parts of the Universe and others not. Also realise that even if your question did make sense nobody would know the answer as our current theories break down sufficiently close (in time) to the Big Bang.

11. Oct 14, 2015

### Helios

I am not claiming that the Big Bang is a single place. Say, at time after the Big Bang, day 2 or something, a photon is emited and never obstructed, never messed with, could it reach me?

12. Oct 14, 2015

### Orodruin

Staff Emeritus
That depends on where it was emitted.

13. Oct 14, 2015

### Helios

Let me ask it this way: What is the oldest photon that I can possibly see? ( ideally, no obstructions, no ricochet )

14. Oct 14, 2015

### Orodruin

Staff Emeritus
If you could somehow find a massless particle which did not interact with the rest of the contents of the universe you could see them as far back as the Big Bang. Now, how you would ever detect such a particle is a mystery. The closest thing you could get would be primordial gravitational waves.

15. Oct 14, 2015

### Helios

I would readily agree were there a static universe, however in an expanding universe, that expanded fast enough, it would seem that the earliest photons would never reach me.

16. Oct 14, 2015

### Orodruin

Staff Emeritus
This is not a matter of agreeing or not. It is a direct implication of current cosmological models (albeit extended beyond their domain of applicability). There is simply no other way to state it other than saying that you have a misconception about how the expanding universe works.

17. Oct 14, 2015

### Jorrie

I agree with Orodruin, you need to learn the basic cosmology model before the answers to these questions will make sense to you.
I have a related question of my own: if we could observe photons (or other massless particles) emitted say 1 second after t0, their black body temperature would be of order 1010 °K.* Would we still observe them at around 3 °K, but just with a vastly greater redshift?

* I quote this rough figure from George Smoot's "Wrinkles In Time".

18. Oct 14, 2015

### Orodruin

Staff Emeritus
They would be even colder. At 1010 K, you still have enough energy for electron-positron pairs to be produced. Once the typical energy falls below the electron mass, annihilations of electrons and positrons will release energy to heat up the matter in the Universe (the entropy in the electron-positron gas will be transferred to the remaining particles - mostly photons), but a species which was decoupled before that will of course not be heated and so will be cooler than the matter sector.

As it so happens, we do have particles which are massless (for those purposes -- this year's nobel prize was actually for showing that they are massive) which did decouple around 1 s after the Big Bang, namely the neutrinos. The Big Bang model does predict a cosmological background of neutrinos with a temperature of around 1.95 K. Of course, observing this background would be extremely difficult as neutrinos with those energies would interact very very rarely. This is the reason neutrinos are thought to be the second most abundant (known) particle in the Universe (after photons).

Also, note that the unit Kelvin is denoted as K, not °K (i.e., it is Kelvin, not degrees Kelvin).

19. Oct 14, 2015

### Jorrie

Thanks Orodruin. So, if somehow photons could have made it through from that time, they would also have some 1.95 K average temp now? How would one have determined the photon redshift for that time? I assume that if t is extrapolated to zero, the redshift would increase without limit.

20. Oct 14, 2015

### Orodruin

Staff Emeritus
It would, the redshift is directly related to the scale factor of the Universe as $1+z = a_0/a_1$ where $a_0$ is the scale factor today and $a_1$ the scale factor at the time of emission.