Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Viewing the beginning of the universe.

  1. Jun 22, 2011 #1
    I dont know if this is the right topic to post this, so if it isn't please point me in the right direction.

    Ok so I was watching a video on SixtySymbols and they mentioned a picture from the Hubble called Deep Field. And talked about how in that image you are able to see objects from around 10 billion years ago(he just kinda pulled a number from his head but he was just making a point). So my question is since basicaly the farther away you look the farther back in the past you look, does this mean that you can look out far enough to see the beginning of the universe; and if so, wouldnt this assume that the universe is not infinite?
     
  2. jcsd
  3. Jun 22, 2011 #2

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    You can't see the beginnings of the WHOLE universe. You can only see a chunk of the early universe

    estimated to be a large typical sample of how it was about 380,000 years after the start of expansion.

    We can't look any deeper back because before that the hot gas was not transparent enough.

    ===============

    There are skymaps of the U that we can see as it was in year 380,000 (or the point in time when it became transparent enough for the light to escape. when the hot glowing gas became cool and thin enough that it "cleared" and visibility improved.

    These skymaps are called CMB maps or CBR maps
    for "cosmic microwave background" or "cosmic background radiation".

    Try googling "CMB wikipedia" or something like that and get a picture of the map. It is a mottled blue and red oval. The blue and red are colorcode for the temperature. It is blotchy because the temperature varies slightly depending on what part of the sky.

    CMB mapping is the main way we have to study the very early universe. Back earlier than year 380,000 involves theoretical models and guesswork because we can't see back earlier into the hot fog.

    There weren't any galaxies or stars or stuff back then. Just nearly uniform hot glowing gas, with interesting blotchy ripples of temperature and density variation (those ripples were the beginning of structure and pattern that eventually evolved into the present structure and pattern including us.)

    If you want to understand more the basic concept you have to wrap mind around is REDSHIFT which is the factor by which space and wavelength has stretched out during the time the light was traveling to us.

    The redshift of the CMB radiation is about 1090 or 1100 which means that distances have stretched out by a factor of 1100 while the light was traveling, and also that the wavelengths of the light---the hot gas glow---have stretched out by the same factor of 1100.

    So what once was fierce orange light from 3000 degree hot gas is now faint cool radiowaves or microwaves. The stretching both of distances and of lightwaves in transit is the central idea you have to get used to start understanding cosmo.

    If you can't find a Wikpedia map of the Cosmic Microwave Background Radiation, then ask us to help.
     
    Last edited: Jun 22, 2011
  4. Jun 22, 2011 #3

    cepheid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    marcus has mentioned a very important practical limit on how far away the oldest light we can ever see comes from. Just to add to what he said, I thought I would point out that even if the early universe hadn't been a hot, dense, and opaque plasma, we still wouldn't be able to see all the way back to the beginning. There is a very important theoretical limit on far we can see, and it comes from the fact that the universe has a finite age, and light travels at a finite speed. As a result, today, there is a distance beyond which photons have not yet had time to reach us in the age of the universe. In other words, you cannot see any light from objects beyond this distance (even in principle), because the photons emitted by those objects have not yet arrived. This theoretical upper limit on how far we can see is called the horizon distance, and it defines the radius of the observable universe.

    If memory serves, the horizon scale is approximately 46 billion light years. You might expect it to be 13.7 billion light years, and that would have been true in a static universe. However, the universe is expanding, and as a result the horizon scale is larger than the light travel time would suggest.
     
  5. Jun 22, 2011 #4

    marcus

    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Good point. Thanks Cepheid!
     
  6. Jun 23, 2011 #5

    Nabeshin

    User Avatar
    Science Advisor

    I'd just like to point out that while the ionization of the early universe puts an upper limit on how far back in time we can observe, we should note that it is still possible to make direct observations of the universe before this epoch. Specifically, by observing neutrinos you can probe the universe up until it was about 1 second old. Then, theoretically, you should be able to detect gravitational waves from the early universe, which should get you basically to the time when the universe was a planck time old.
     
  7. Jun 23, 2011 #6
    Thanks for the replies Marcus and Cepheid. Your descriptions helped me understand a lot more.

    How is it that we are able to seperate the CMB's radiation and the rest of the universe's radiation and determine where/when it came from?
     
  8. Jun 23, 2011 #7

    cepheid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yeah, actually I was wrong above when I said that the horizon issue meant that we could not see all the way to the beginning in principle. In *practice* we can't because we can't see photons emitted before about 380,000 years after the big bang, and present technology prevents us from detecting primordial neutrinos or primordial gravitational waves. But in *principle* IF we could "see" (meaning "detect signals") all the way out to the edge of the observable universe, then we would be seeing signals that were emitted as far back as the beginning of the universe. So, the horizon doesn't limit how far back in time we can look, only how far out in distance. Thanks for making me aware of my mistake.
     
  9. Jun 23, 2011 #8

    cepheid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    That's an excellent question. To a certain extent, our telescopes don't distinguish. An experiment like WMAP maps the whole sky at microwave wavelengths, and so any radiation that lies within the microwave "bands" (wavelength ranges) to which our telescopes are sensitive will be detected, regardless of source. We don't have to worry about radiation from sources that is not at microwave wavelengths, such as gamma rays, X-rays, UV, visible light, near infrared, and longer radio wavelengths, since we've designed our telescope to be sensitive only to certain narrow ranges of the EM spectrum corresponding to microwaves. Nevertheless, microwave radiation from other sources is still problematic, particularly for CMB observations. Normally in astrophysics we are interested in some foreground source, and any background emission at the same wavelengths just acts as noise that reduces our sensitivity. With the CMB, however, it's the smooth (and faint) background emission we're interested in, and all other sources are in the foreground and are much stronger! There are a few things that aid in foreground mitigation/removal though:

    1. The CMB Isotropy

    We see the see CMB over the whole sky i.e. it comes at us from all directions. In contrast most other microwave sources are localized. For instance, the dominant source of foreground emission is thermal emission from dust within our own galaxy. That tends to be confined to a narrow band across the sky corresponding to the Galactic plane. If you don't need to map the whole sky, you can look away from the Galactic plane where foreground emission is lower. If you do need to map the whole sky, you need to get fancy and try to remove the dust emission. That could be aided by...

    2. The CMB Spectrum

    The spectrum of the CMB corresponds to an almost perfect blackbody radiator with a temperature of about 3 K. In fact it's one of the finest examples of a blackbody in nature. There aren't any other astrophysical sources of emission that are 3 K blackbodies and appear distributed over the whole sky. In fact, the spectrum of thermal dust emission from the Galactic plane will appear quite different, if nothing else because the dust is not as close to being a perfect blackbody emitter. Furthermore, it has a much higher temperature than 3 K (I think that even the coldest and densest dusty molecular clouds we've observed in our Galaxy are ~10 K), so the shape and amplitude of the dust spectrum in the bands in which you're observing is going to be totally different from the CMB spectrum. If you can model the spectra of all the various components that contribute to the emission you're observing, you can then try to separate them from each other. Of course, the much higher dust temperature probably leads to much stronger emission in general, which contributes further to the problem (if you look at the unprocessed WMAP CMB map, you'll see how much brighter the emission in the Galactic plane is than anywhere else). Bottom line: foreground subtraction is a problem that every CMB experiment has to deal with.

    EDIT:

    3. The CMB Redshift

    You also asked how we know when the emission comes from. We can figure out theoretically what the redshift of the CMB photons ought to be because we know what the radiation temperature ought to have been at the time that the process that produced those CMB photons occurred. This process is known as recombination and basically refers to the universe cooling to the point that protons and electrons could combine to form stable hydrogen atoms for the first time. This process would have occurred when the radiation temperature was around 3000 K. Note: the transition from a plasma to neutral gas is essentially what caused the universe to go from opaque to transparent -- for the first time, photons could stream freely through space without having to worry about constantly bumping into charged particles. That's why we see the CMB photons coming at us from all directions. Anyway, the point is that the present measured radiation temperature of the CMB is around 3 K (vs. 3000 K when it was emitted), and since radiation temperature scales linearly with redshift, we know that the CMB photons have been redshifted by a factor of about 1000. I imagine that if we can measure the redshift of some foreground source of emission and determine it to be much much less than 1000, then we know that that is indeed a foreground source and not CMB. However, I sort of just made that last part up based on common sense, so take it with a grain of salt.
     
    Last edited: Jun 23, 2011
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Viewing the beginning of the universe.
Loading...