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Neil deGrasse Tyson's Observable Universe

  1. Mar 13, 2014 #1
    In Dr. Tyson's first episode of Cosmos he said "...there are parts of the Universe that are too far away. There hasn't been enough time in 13.8 billion year history of the Universe for their light to have reached us."
    Since the remnants of the Big Bang has a redshift over 1000 and large redshifts are farther away than small redshifts, what parts of the Universe is Dr. Tyson referring to whose light(or any other electromagnetic radiation) hasn't reached us yet?
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  3. Mar 13, 2014 #2


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    Yeah, I think that was a poor statement, since the CMB (aka the "surface of last scattering") is the oldest electromagnetic radiation that CAN ever reach us and it already does. The only thing that can reach us that is older than that is neutrinos, and possibly gravity waves, from earlier (but no more than 380,000 years earlier which is a pretty trivial amount compared to 14 billion years)
  4. Mar 14, 2014 #3


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    The CMB is a special case scenario, as it originated everywhere as temperature cooled and the universe became transparent. This why it arrives from all directions. And the CMB that arrives tomorrow will on average have traveled further and longer than what arrives today.

    If you define the observable Universe as what we're able to see, it's dynamic and continues to grow, and isn't the same as the Earth's cosmological horizon. Even objects that are now receding faster than c can become visible to us. Here's a snippet from an earlier Cosmology thread:

    EDIT: Stepped out on the patio to look at the stars and reflect. I think I understand the points both of you were making. Words are so important. Saying "hasn't been enough time in the 13.8 billion years" is not entirely true - "will never be able to reach us" would be more appropriate for anything that was further than 13.8 billion light years in today's geometric distance (years since last scattering)

    I was more annoyed that he used the word "explosion" multiple times while describing the BB, although he did finally mention expansion, but it seemed like an afterthought. But I do feel that he's doing a great job of explaining in so few words. You know how long PF threads can get to drill into all of the details. :)
    Last edited: Mar 14, 2014
  5. Mar 14, 2014 #4


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    It's all about the ratings. Dr. Tyson is appealing to popular perception, not scientific rigor. Note that he never mentioned his assertion the universe arose from a point smaller than an atom only applies to the observable universe. I still think it was a good presentation.
  6. Mar 14, 2014 #5


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    I'll try to answer directly just this question which you asked. I think the current consensus model universe is probably pretty accurate, and its all I have to answer with, but nothing is perfect, numbers can always be revised as we find out more, estimates of the age and future of the universe can change, so this is a provisional answer.

    Light from matter that is NOW some 45 billion LY from us is currently reaching us.
    (see the 45.332 in the table?)
    That is the CMB light. The matter that shone that light USED to be much closer when it emitted the light (over 1000 times closer, only some 41 million LY). That was back around year 373,000 (do you see the 0.000373 in the table?)

    Light from matter that is now MORE than 45 billion LY from us has NOT YET reached us.
    (see the 45.332 ?)

    But if it is from matter that is now CLOSER than about 62 billion LY it WILL eventually get here.
    (see the 16.328 in the far distant future? 45.332+16.328 ≈ 61.66 ≈ 62)

    However light from matter that is now MORE than about 62 billion LY from us will never make it here.

    This table is made by a calculator called "Lightcone" that embodies the standard cosmic model.
    It's online and free, you can use it if you want.
    [tex]{\scriptsize\begin{array}{|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|} \hline a=1/S&S&T (Gy)&D_{now} (Gly) \\ \hline 0.001&1090.000&0.000373&45.332\\ \hline 0.003&339.773&0.002496&44.184\\ \hline 0.009&105.913&0.015309&42.012\\ \hline 0.030&33.015&0.090158&38.052\\ \hline 0.097&10.291&0.522342&30.918\\ \hline 0.312&3.208&2.977691&18.248\\ \hline 1.000&1.000&13.787206&0.000\\ \hline 3.208&0.312&32.884943&11.118\\ \hline 7.580&0.132&47.725063&14.219\\ \hline 17.911&0.056&62.598053&15.536\\ \hline 42.321&0.024&77.473722&16.093\\ \hline 100.000&0.010&92.349407&16.328\\ \hline \end{array}}[/tex]

    To use the calculator yourself, click on
    You will automatically get a table like the one shown but with more columns containing additional information. I pared down what I copied here---eliminated columns of information to make ithe table simpler to read. If you click on the link you will get the same numbers but a bunch of other stuff as well. Click on "column selection and definition" and hover the mouse over the blue "info" buttons to get popup explanations.)

    To get explanations from us, ask questions here in this thread. I'll check back in case you want help understanding the table. Anyway that answers the actual question. The matter whose light has NOT YET reached us BUT WILL EVENTUALLY is precisely the matter which NOW, if you could freeze the expansion process to make it possible to measure the distance without it changing meanwhile, is between 45 and 62 billion LY from us.
    that is a heck of a big piece of the universe! So there is a lot that we have not seen yet whose light will eventually get here and give us some clue about it.
    This takes account of the current estimates of the rate expansion is accelerating, which party explains the 62 billion LY limit. If interested in understanding more, it would be natural to ask for more explanation. I just gave you the bare-bones answer to what uyou asked.
    Personally I'm suspicious of mass media pop-science, when they "talk down" for wide audience to expand their network ratings they oversimplify and potentially cause huge confusion in the public's mind, so beware :biggrin:
    Last edited: Mar 14, 2014
  7. Mar 14, 2014 #6


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    It could take an entire episode just to explain the observable universe. Correcting the public misperception of the big bang as an 'explosion' would likely take another episode. I question the value of the time necessary to properly elaborate these concepts on television when there is so much to tell. Any truly curious person can easily research these subjects on the net.
  8. Mar 16, 2014 #7
    Marcus I am confused. I think I understand that in the year 373,000 light was emited from 41 million LY away that we now know as Cosmic Background Radiation. Are you saying that there was matter much farther out than 41 million LY when CMB was emitted that is now 62 billion Ly away NOW.
  9. Apr 12, 2014 #8
    The CMB is coming to us from everywhere. It is right next to us. WMAP and Planck are showing us the microwave radiation released at recombination that is right next to us and in us and going through us. It is NOT at the edge of the observable universe. Well it is, but that is not what they are seeing. Those satellites (they arent telescopes, they are satellite-thermometers) arent looking at the edge of the universe 13.8 billion years ago. No telescope has looked back that far. Think about it. Hubble is the most powerful telescope ever put into space. Not even it can peer so deeply that it will hit that opaque wall existing at 380,000 years after the BB, which merely remains the theoretical limit to which we could see nit the actual limit. The CMB we see is all around us and flowing everywhere around and through us. Planck is measuring its temperature as it senses the temperature of the space around it. We have not actually "seen" that opaque wall of recombination. Yet. Make sense?
    Last edited: Apr 12, 2014
  10. Apr 12, 2014 #9


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    Yeah, but the CMB photons did, in fact, originate from the last-scattering surface 380,000 years after the big bang; just like the photons from Andromeda that hit Hubble's mirrors originated 2.5 million years ago.

    True, the CMB is all around us, but it's also the oldest light we can see. Hubble does not see "further back in time" than Planck.
  11. Apr 12, 2014 #10
    Great point. I wasn't really thinking of it that way but you're absolutely right of course. That's an interesting thing to think about. Planck is detecting older photons, actually the oldest, the ones originating in the BB. The key difference between Hubble and Planck is that Plank is detecting a certain kind of photon, those with a characteristic temperature of 2.7 plus/minus .002 K. It is fine tuned to a very specific and narrow frequency, and since the only structure visible at that range is the tiny temperature variations of the CMB, it doesn't need to be large. Hubble's large mirrors are of course detecting photons of a much wider temperature range and thus is detecting more intricate structure, and it's size is the key difference, as it is able to collect far more photons over a much wider area in order to make out that structure.
  12. Apr 12, 2014 #11


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    The evidence strongly suggests CMB photons originated from the surface of last scattering 13.7 billion years ago. The concept of viewing any photon source older than the CMB is confusing, and pretty meaningless. You can plug as big a number as you wish for z into a cosmological calculator, like Jorrie's, and distance NOW will max out around 46642 Mly - well short of the 62000 Mly that marcus noted. So, where does this 62 Bly year value originate? It is derived from the integral form for computing redshift distance. All this tells us is in principle, due to expansion, it is forever impossible to observe anything [irrespective of how logical its existence may be] at a distance NOW exceeding 62 billion light years. It's just one of those curious calculations that amuse cosmologists while awaiting grants. In the 'real' universe, we will never observe anything beyond the CMB - at least not in the EM spectrum. There are no stars, much less galaxies, lurking behind that curtain whose photons will someday reach us.
  13. Apr 13, 2014 #12
    This makes me think of another question. The spectrum of the cmb includes those photons with an average wavelength corresponding to a 2.725 k temperature. But thinking through this my understanding is that at the time of decoupling there would have been many wavelengths of light emitted. First you have all the photons from the BB, corresponding to the 1B to one ratio of photons to baryons if I remember correctly. They stream free after they no longer scatter. Then you have the photons that are given off by the electrons as they drop to lower orbitals around the hydrogen nucleus, which vary in wavelength as they jump down to the ground state. These wavelengths would vary across the UV, visible, and IR wavelengths, correct? So there would be many different wavelengths given off at decoupling that would then be redshifted with the expansion of the universe. Are all of these included in the 2.725 spectrum? They have to be I would think. So how do those different component wavelengths break out in the cmb spectrum? The vast majority of those photons (1B to one) would be from the original scattering spectrum just prior to recombination, then there would be a far smaller amount given off as the electrons cascade to the ground state. Are those a different component in the cmb? Do scientists break this out? Could they? I imagine they can but have never seen detailed information that would answer my question. Thanks.
  14. Apr 13, 2014 #13
    good question, also one that is difficult to answer, in order to understand you need to also understand the thermodynamics involved. in regards to number of species of particles, entropy density, energy-density to pressure relations. energy-momentum tensor relations etc. For all that I recommend the following.

    http://arxiv.org/pdf/hep-ph/0004188v1.pdf ASTROPHYSICS AND COSMOLOGY"- A compilation of cosmology by Juan Garcıa-Bellido this article delves into the metrics involved.

    http://www.wiese.itp.unibe.ch/lectures/universe.pdf :" Particle Physics of the Early universe" by Uwe-Jens Wiese this one also does so but doesn't specify time periods clearly as he refers to the related temperatures more so.

    http://arxiv.org/abs/0711.3358 "Physics of the Intergalactic medium" this one is not a training book and is very technical but it does have a very good coverage of many of the considerations including analysis.

    for actual textbooks I recommend Scott Dodleson's "Modern Cosmology"

    As Chronos mentioned much of our understanding of decoupling derives from our understanding of particle physics rather than direct observation.

    edit: I should note this is an area of Cosmology I myself am currently studying so others are better suited to explain in detail the metrics involved. I wouldn't want to make any mistakes with examples
    Last edited: Apr 13, 2014
  15. Apr 13, 2014 #14
    Wow thanks for the links. I will read them over and attempt to comprehend. A lot of this is way beyond me but ill learn as much as i can.
  16. Apr 13, 2014 #15
    no problem, I should also recommend a couple of entry level textbooks. Not sure If your willing to buy them but they are invaluable.

    "Introduction to Cosmology" Barbera Ryden she does an excellent job on the FLRW metrics and single/multi component universes. She does get into CMB related metrics as well. Though in a simpler format.

    "Introductory to particle physics" by Peter Griffith I found this book invaluable to understanding particle physics and what it means in terms of particle properties and thermal equilibrium. Its invaluable as he keeps the metrics involved to a simple and easy to understand format.

    http://cosmology101.wikidot.com/main my signature has some decent article in regards to cosmology though not so much on CMB, still looking for easier to understand and non controversial references for that section when I have time. You will notice also the lightcone calculator link is also on that page
  17. Apr 13, 2014 #16


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    The reference is "Introductory Elementary Particle Physics" by David Griffiths. He also has an excellent QM textbook for undergrads.
  18. Apr 13, 2014 #17
    D'oh your right, I should have checked my copy too many articles and books lol.
  19. Apr 14, 2014 #18


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