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Understanding the universe in real time

  1. Mar 10, 2014 #1
  2. jcsd
  3. Mar 11, 2014 #2


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    Staff: Mentor

    We are not looking back 30 billion years. We are only looking back 13.1 billion years. The difference is due to the expansion of the universe moving the galaxy away from us while the light that we see was still in transit.
  4. Mar 11, 2014 #3


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    To say what Drakkith said, another way, is this: the time difference is that while the light was emitted 13B years ago, it had to travel through a lot more that 13B light years of space, because space has been expanding, so while the light is always traveling at c locally, it is heavily red-shifted by the time it gets to us
  5. Mar 12, 2014 #4
    An "amature" should know the difference between 30 and 13. The Big Bang happened (we believe) about 13.8 billion (thousand million) years ago. Therefore no light is older than 13.8 billion years. During the first fractions of a second of the Big Bang, (we believe) the Universe expanded at many times the speed of light so that we estimate that the Observable Universe now has a radius of about 46 billion light-years (centered on us, of course). It is 100 years since Einstein showed that terms such as "real time" are meaningless. This isn't the place for a course on relativity as it applies to cosmology, an ok place to start is wikipedia, although there are many good video introductions on the web, too. So, the Universe is about 14 billion years old. We have assumed for decades that the Universe is isotropic and homogenous. These assuptions have been tested and so far have held up, but we're still "new" at this stuff so tomorrow might find us changing our minds as we learn more about the Universe. Keep in mind, we have a fairly good grasp (we think) of only about 5% of the "stuff" (energy) that the Universe is composed of - meaning that we're 95% "in the dark" with lots to learn. But so far, we think the Universe looks pretty much the same (on "average") any direction you look. Which is to say if you want to know what the Universe looks like "now", look around you. Our Galaxy is only about 100,000 light years across, so looking out at the stars in the sky is seeing the whole Universe as it is "now". What's so cool about looking far into the past is seeing how the Universe was then when stars were new and galaxies hadn't yet formed. We think that the early Universe looked a LOT different than it does today.
  6. Mar 18, 2014 #5
    thanks for pointing out my error. however, that was not my question. let me be clear. if we observe the universe as it was 13bn years ago... what does is look like today, 13b years after the BB?
  7. Mar 18, 2014 #6


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    There's no way to know really, since we can't see it. We infer however, with some very reasonable assumptions, that it has evolved in the same way that it appears to have evolved based on what we CAN see right now even though what we are seeing varies from near-real-time very locally to 13B years in the past in the case of things like the Hubble Deep Field.

    For example, some of the galaxies that we see in the Hubble Deep Field are not yet well formed because we are seeing them relatively shortly after the singularity but we assume that they have by now evolved just to be like the nearer galaxies that we can see in the less distant past.
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