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Looking back in time to the big bang

  1. Mar 10, 2010 #1
    This may seem a silly question to some. but I am asking it for a reason which I will expand upon after someone explains to me the how.

    How do we look back in time to the big bang?

  2. jcsd
  3. Mar 10, 2010 #2


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    WMAP is our best effort to date. CMB photons are observed to arrive at z~1090. Applying our most accurate estimate of Hubble expansion [Ho], we derive an age of about 13.6 Gly. Big bang nucleosynthesis pushes the time of the big event back another 400 million years.
  4. Mar 11, 2010 #3
    Although we cannot look directly at the big bang at say 10 seconds right after the event, using the WMAP and COBE satellites as Chronos mentioned, we are able to observe the time at which photons and baryonic matter decoupled. Once decoupled the photons were able to travel freely and allow nucleosynthesis to take over. This time correlates to around 13.6 Gly as Chronos has mentioned as well.

    We are able to confirm this time line by setting lower limits on what it could truely be by judging the oldest stars within the Universe, namely white dwarf stars and main sequence stars. These stars follow very methodical routines with respect to temperature, luminosity, and age, giving us an accurate limit to how young the Universe could be. I hope that helped.

  5. Mar 12, 2010 #4
    Originally when I posed this question, it was my belief that the universe was expanding at the speed of light, this bought about my question. because if it was, any light emitted soon after the big bang would be far gone and we could never see it therefore we could not "look back in time" I am now starting to understand that we are in fact not looking at the big bang but the residual 'glow' left behind by the big bang, the CMBr.

    As usual it raises more questions than it resolves :(

  6. Mar 15, 2010 #5
    Please ask away and we will try to help sort things out for you.
  7. Mar 15, 2010 #6


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    Maybe your problem comes from imagining expansion the wrong way.
    Have a look at the Lineweaver article about Misconceptions people often have about the big bang. I keep the link in my sig.

    One common misconception is that it was an explosion of matter out into empty space. Lineweaver and Davis explain why that's the wrong picture. The article has helped a lot of people. It is clearly written and well illustrated (like many Scientific American articles).

    Your question is about the standard model that cosmologists use. According to that model, the early universe had no empty space. It was uniformly filled with glowing hot ionized gas. Or if you go back even further, a hot soup of subnuclear particles. My point is that it was uniformly filled.

    And it wasn't transparent. Like the surface of a star it was glowing too brightly to be transparent, glowing gas scatters any light that tries to get through it.

    It didn't get cool enough to turn transparent until an estimated 380 thousand years from the start of expansion. So the oldest light we see is from then.

    If it had turned transparent earlier, then we would be seeing even older light.

    There never was a problem of light somehow "getting away" from us, or "outracing" expansion. Matter and space are coextensive. With matter (and light) more or less uniformly distributed throughout. The whole picture expands together.

    So each part is always getting ancient light from some other part.

    I think the best way to imagine it is to start with a simplified 2D analogy.
    Google "wright balloon model" and watch the computer animation. In that analogy all existence is concentrated on the surface of an expanding sphere. The CMB photons of light are shown as little wigglers always traveling at a fixed speed across the 2D surface. The galaxies don't change their position in terms of longitude and latitude, but they get further apart. If you watch it a few times you will get some intuition for the standard model.

    Of course space and matter could be coextensive and both infinite! That's a possibility. We don't know yet which case we are looking at. Infinite space with matter uniformly distributed throughout, or a finite version analogous to the 2D surface of a sphere. For a lot of purposes it doesn't matter how that turns out, though.

    The Lineweaver Davis article was originally published in the Scientific American but it is so good that it has been used in Princeton astronomy courses. So they have a nice online version at princeton.edu
    http://www.astro.princeton.edu/~aes/AST105/Readings/misconceptionsBigBang.pdf [Broken]
    Last edited by a moderator: May 4, 2017
  8. Mar 16, 2010 #7
    Nope don't have a problem. my question was posed before I worked out the answer.
    Just after the big whatever it was. there was a seething cauldron of hot stuff it was about (off the top of my head) 40 odd Million light years across. After the great ionisation festival (I think it was a festival) light started to move across the universe which was now cooling. it took about 45 Billion light years to get to us. but it seems that its a trick of expanding space(/time) so in actual fact the real age of the universe is about 13.6B light years.

    Oh and theres nothing outside of space/time so how can one expand into it?


    p.s. I hope you realise I am having fun and not being rude or anything. I have learnt a lot in the last week or so :) So I hope I am somewhere near the mark.
  9. Mar 16, 2010 #8


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    Sounds to me very near the mark. The rest is minor editing--you could say quibbling.
    Say we measure time in years, and distance in lightyears.

    Then it sounds clueless to say "the age of the universe is about 13.6 B light years".
    Instead say "the age of the universe is about 13.6 B years." Simply.

    You really shouldn't express the age of something, some interval of time, in "light years".
    But some folks do that. It is a common goof. Hope you aren't offended by this quibble-editing.

    And I like the "seething cauldron of hot stuff" image. It strikes me as really accurate visualization. The image would apply to right before it turned transparent. That moment is estimated to have occurred right around year 380,000 of the expansion. At that moment the temp is about 3000 kelvin, like the surface of a star which is somewhat redder and cooler than the sun, but still very hot.
    The seething cauldron of hot stuff is expanding rapidly and therefore cooling and about that time it suddenly turns transparent and the light gets loose (like when a fog clears) and the light starts its run.

    the CMB light that we are seeing now started from matter that was around 40 million ly (as you estimated!) away from us, and has been traveling towards us for some 13.6 billion YEARS (not lightyears). And the matter it started from is now about 45 billion ly from us.
    Last edited: Mar 16, 2010
  10. Mar 17, 2010 #9
    I asked this a little while ago but noone answered it seems relevant here so I will try again . Before baryogenesis did all paricles move at relativistic speed and if so does that therefore suggest that time was stationary from our perspective . This being the case is it in essence meaningless to ascribe any time referance to big bang and inflation prior to the creation of slower than light particles ?
  11. Mar 17, 2010 #10
    Tilly, during this hot soup before baryogenesis, particles were definitely moving at relativistic velocities due to the immense temperatures. This is different than traveling at c, which is prohibitted for anything possessing mass. The term relativistic velocities simply means velocities approaching that of c, which is where relativistic effects are present such as time dilation and length contraction, but they still have the time dimension in their frame of refrence. Hope this helps.

  12. Mar 17, 2010 #11
    thanks Joe two follow up oints /questions then . If pre baryogenic particles were ascribed with mass and therefore moved at speeds less than but close to c does that suggest time moving very slowly relative to ourselves ? Can we suggest that prebaryonic particles possess mass other than that derived from energy levels ?
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