When Did We Last Get a Glimpse of the Big Bang?

[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?

 

[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.

 

[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)

 

[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?

 

[phinds]

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?

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".

 

[bapowell]

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?

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.

 

[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?

 

[Orodruin]

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.

 

[Helios]

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

 

[Orodruin]

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 realize 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.

 

[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?

 

[Orodruin]

That depends on where it was emitted.

 

[Helios]

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

 

[Orodruin]

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.

 

[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.

 

[Orodruin]

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.

 

[Jorrie]

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.

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".

 

[Orodruin]

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).

 

[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.

 

[Orodruin]

Thanks Orodruin. So, if somehow photons could have made it through from that time, they would also had 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.

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.

 

[Jorrie]

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.

Cool, I knew that bit, but what I'm not sure of is this: does the early plasma (matter) temperature scale inversely with the scale factor? And then, as you have said, earlier particles could have been colder than the plasma. Is there a way to model those particle temperatures?

 

[newjerseyrunner]

At super high energy, the very concept of light doesn't even exist because the electromagnetic force hadn't separated from the other forces. After that point, the universe was filled with a dense plasma which light could not penetrate, any time a photon tried to move, it immediately got absorbed by something else. It took a full 380,000 years for the universe to cool enough for atoms to start letting light propagate freely in the universe, that's the start of light. You can't see anything before that. You know how the creation myths say that at first there was darkness then "let there be light." That's actually how it happened, there was no light for all that time, at least not in the sense that you are familiar with.

There were neutrinos though, but I don't think there is any way to use them to peer past the CMB. The oldest photons are the age of the universe minus the recombination epoch.

 

[Orodruin]

There were neutrinos though, but I don't think there is any way to use them to peer past the CMB.

There are several possible experimental approaches to detect the C$\nu$B. The big question being whether or not it will be technically possible to make them sensitive enough.

Anyway, I have seen this question more as a question regarding the cosmic horizons than using actual photons.

 

[Helios]

I get now that an observer with fictional vision of a fictional massless particle ( attributed to the Big Bang ), would never lose sight of the Big Bang no matter how long he endured, despite the expansion of the Universe, though the redshift will increase.
I could have been spared the diagnosis of mental shortcommings. This isn't difficult and I now understand the answer to my question. Bringing up "direct implication of current cosmological models" and "how the expanding universe works" really exaggerated the profundity of this issue.

 

[Hornbein]

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.

It would depend on where you were. Maybe they would reach you, maybe not.

 

[ChristianG]

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.

You say the Big Bang happened everywhere. I have been lead to believe that it was a singularity that exploded which in turn affected everything. Rather than the singularity appearing in space, space came from the singularity. So, are you saying because the Big Bang was the entirety of space it occurred everywhere . Can you please elaborate on that so I understand it. Or point me somewhere that I can read on it.

 

[Bandersnatch]

You say the Big Bang happened everywhere. I have been lead to believe that it was a singularity that exploded which in turn affected everything. Rather than the singularity appearing in space, space came from the singularity. So, are you saying because the Big Bang was the entirety of space it occurred everywhere . Can you please elaborate on that so I understand it. Or point me somewhere that I can read on it.

Start with the universe at the present day. It is of unspecified size - might be just very large, or even infinite. The relative positions of large-scale structures in the universe remain unchanged as it evolves, only distances change.

Measure any number of distances between such large-scale structures in the universe. As you go back in time, those distances will be shorter and shorter by the same factor per unit time (which translates to higher average density in the universe). As you go further in time, those distances will be longer and longer (which means density will go down).
That's the Big Bang theory in a nutshell. It happened 'everywhere', since all distances are affected, regardless of where you measure them, in how large a universe (including infinite), and where you're standing.

You can calculate how fast the distances grow shorter as you go back, and that they all reach 0 in length (=infinite density), in a finite time. That's the singularity. It's a singularity in time, not in space.

You can't talk about space before singularity, since space is a separation between objects, and it's already 0 everywhere. In this sense, space 'came from' the singularity.

However, consider how you can't divide by 0. One way of understanding it, is that the function f(x)=1/x has a singularity at point 0. That is, its value goes to infinity (or minus infinity) as you approach zero. But it hardly means that the values the function depicts 'came from' the singularity at point 0.

One more thing to add, is that no cosmologist (I know of) considers the cosmological singularity an actual physical feature of the universe. Pretty much everyone sees it as an indication that the theory just doesn't correctly describe the very early moments in the history of the universe. Much like you can't divide by 0, you can't use use the BB theory at time 0.

 

[Gaz]

Guys when the temp cooled and the universe became transparent i read around 57,000 years after the big bang what would the wavelength of the CMB be then?

 

[phinds]

Guys when the temp cooled and the universe became transparent i read around 57,000 years after the big bang what would the wavelength of the CMB be then?

It was 400,000 years after the singularity, not 57,000 but I don't know the wavelength.

 

[Bandersnatch]

CMBR is not a single wavelength, but a continuous blackbody spectrum. Its current spectrum is consistent with a 2.7K temperature blackbody. At recombination the corresponding temperature was 1090 times higher. If all you want is the shift of the wavelength of the radiaton peak, you can calculate it from Wien's displacement law.

 

[PeterDonis]

when the temp cooled and the universe became transparent i read around 57,000 years after the big bang what would the wavelength of the CMB be then?

As phinds says, this event, which is called "recombination" (see below), was a few hundred thousand years after the Big Bang. It was also the event at which the CMB was formed; "the universe became transparent" was the condition that had to be met for the CMB to exist--before that, radiation emitted by the matter in the universe was quickly re-absorbed because the matter was ionized plasma. "Recombination" means the electrons and ions in the plasma came together to form neutral atoms; that was what made the universe transparent to EM radiation and allowed the CMB to exist.

The wavelength of the CMB at this point, when it was first formed, was determined by the temperature at which recombination occurred, which was a few thousand degrees Kelvin. That temperature determined the average energy of the radiation that formed the CMB when the universe became transparent, and the average energy in turn determined the frequency and wavelength of the radiation. We can estimate what that wavelength was by measuring the redshift of the CMB, which turns out to be about 1000; so the wavelength of the CMB when it was formed was about 1000 times shorter than its wavelength now. Its wavelength now is about 10 mm, so its wavelength then would have been about 10μm, or about $10^{-5}$ meters.

 

[rootone]

Long wavelength infrared.
I was expecting something more spectacular!

 

[PeterDonis]

Long wavelength infrared.
I was expecting something more spectacular!

Remember that mine was a very, very rough calculation. Also remember that the CMB is not just one wavelength; it's a black-body spectrum at a particular temperature, which today is about 2.7 K, and when it was formed was a few thousand degrees K. The wavelength I quoted was just the wavelength at which the spectrum peaks; there is significant radiation at a fair range of longer and shorter wavelengths as well. When the CMB was formed, there would have been a significant component in the visible range; something like a star rather cooler than the Sun.

 

[rootone]

Thanks, yes I figured it would be something like that, so in principle the recombination event would actually have been visible to some hypothetical human observer.
Probably they could see light/feel heat in the same range as that of the embers of a small wood fire.

 

[bapowell]

Long wavelength infrared.
I was expecting something more spectacular!

Yeah, keep in mind that a quick order of magnitude guess assumes that photon energies would be in the neighborhood of the hydrogen ground state energy, which are in the UV part of the spectrum. So it would be at most that spectacular. In actuality, one finds that the photon energies peak a few orders of magnitude less than UV, into the IR as PeterDonis says.