How old is the oldest light visible from Earth?

  • Because light can only travel so fast, all of the light we see in the sky was emitted at a previous moment in time. So if for example we see a supernova or some other great stellar event, by the time we see it, it maybe long over. That made me kind of curious, what is the most ancient light we can see from earth?



    The universe is supposedly ~13+ billion years old, but we are probably not at the very edge of the known universe so all the light we see is probably less than 13 billion years old. So what is the oldest light we can see? and as an optional follow-up question how do we know the age of that light?



    I guess the light itself may not actually be literally 'old', but its probably obvious what I'm asking here, put another way: what's the longest distance that now earth visible light emitted has traveled to reach the earth? Though that reformation of the question gets kind of tangled with lensing effects.


    Either you ask for a concise summary of state of the art cosmology or you ask which is the farthest away (in light travel dustance) star being naked eye visible. ..

    "but we are probably not at the very edge of the known universe" - We're in the exact middle of the visible universe, since we can see back to the emission of the CMB in all directions.

    @JollyJoker But isn't everything in the exact "middle?"

    There's no edge.

    @DonBranson There *may* be no edge. We have no way of telling. There's little reason to assume there is an edge, but there's also little reason to assume there is no edge. Knowing the limits of our knowledge is important.

    @PhilNDeBlanc Every place is in the middle of its own observable universe, yes

    Are you asking for the oldest detectable electro-magnetic radiation? (which is what everyone seems to be assuming) Or are you asking for the most distant naked-eye object? (which would be the Andromeda Galaxy, a mere 2.5 million light years away).

    Well, I was originally interested in what the furthest directly emitted light was which I guess is from GN-z11 rather than the age of background radiation. I wasn't concerned with whether it was visible with the naked eye or not, just if it was somehow detectable. Of course, some of the premises of my question came from some ignorance of astrophysics, so the precision of my question wasn't spot on. But the selected answer seems to answer everything I was looking for.

    A minor point: your question has an unnecessary geocentric angle. The oldest light appears the same from *everywhere* in the observable universe, not just uniquely from Earth.

  • HDE 226868

    HDE 226868 Correct answer

    4 years ago

    The oldest light in the universe is the cosmic microwave background. Roughly 380,000 years after the Big Bang, protons and electrons "recombined"1 into hydrogen atoms. Before this, any photons scattered off the free electrons in the plasma filling space, and the universe was essentially opaque to light. Once recombination occurred, however, photons were able to "decouple" from the electrons and move through space unimpeded. This relic radiation is still observable today; it has been redshifted and cooled.



    We can detect light from very distant objects, and we have. It makes more sense to talk about distance in terms of redshift; the larger the redshift, the farther away an object is. There are a number of extremely high-redshift objects, some of which have had their measurements confirmed, and others of which have not. Candidates include





    All of these objects would have formed some hundreds of millions of years after the Big Bang, however, so the light we see from them is much "younger" than that of the cosmic microwave background.






    1 I've never liked the usage in this context, as this was the first time they combined; the "re" is kind of misleading.


    I'd argue that "recombined" in this context is not "kind of misleading" but instead downright wrong. But that's not your fault.

    I had to read a lot about redshift to finally find the point where they talk about the expansion of space - I guess that is what you are specifically referring to?

    I'm not sure where the idea comes from that the epoch of recombination is the "first time" the hydrogen has been neutral. Hydrogen ionization was in "ionization equilibrium" prior to that time, and its state of being neutral got "frozen in" during that epoch. What all that means is the timescale for hydrogen to ionize went from being less than the age of the universe at the time to be being more than the age of the universe at an age of about 400,000 years. It also means each electron had been captured and released by protons many many times prior to that. So yes, it's "re"-combination.

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