The Edge of the Physical Universe

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Currently we see to about 15 billions years, determining that to be the edge of the universe.

Could that just be the limit of what we detect can travel?
It experiencing "friction" or other obstacles along it's travels.
 

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  • #2
Chalnoth
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Currently we see to about 15 billions years, determining that to be the edge of the universe.

Could that just be the limit of what we detect can travel?
It experiencing "friction" or other obstacles along it's travels.
Well, that depends upon what you mean by distance. A more accurate statement is that we can see about 13.7 billion years into the past. This is set by the fact that the early universe was opaque: before a certain point, we just can't see. This point is at the emission of the cosmic microwave background.

Now, if we were to "look" into the early universe with something other than light, say something that doesn't interact electromagnetically like neutrinos or gravitons, then we might be able to "see" stuff that happened even earlier. But the hard limit seems to be a few hundred thousand years before the emission of the cosmic microwave background. This limit is set by the way in which space expanded previous to that: according to the simplest models of inflation, inflation wipes out nearly everything that happened before a certain point.
 
  • #3
Chronos
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Neutrino telescope! The observable universe is presently obscured by the surface of last scattering [in the EM frequency] as Chalnoth noted. He is a pretty bright guy, in case you havent noticed.
 
  • #4
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Could that just be the limit of what we detect can travel?
I think you have the right idea: we "observe" out almost 14 billion light years because the radiation from that early epoch, cosmic microwave background radiation, is the major observable available. It's weak but easily detectable with modern technology. It took a long time to reach us during which time the universe was also continuing to expand. We observe only a microscopic piece of the entire universe.

Another possible observable:
It is theorized that long wavelength gravitational waves were produced by early energy density fluctuations as the universe was born. The WMAP satellite has already rejected many big bang models but a new satellite with new detection technology will be required for finer level study to distinguish between big bang and cyclic models of the universe.
 
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Chronos
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We are observationally limited by the surface of last scattering in EM frequencies. A neutrino telescope could take us back to the first second after the Big Event. I question whether our concepts of time are meaningful before 'then'.
 
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  • #6
Chalnoth
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We are observationally limited by the surface of last scattering in EM frequencies. A neutrino telescope could take us back to the first second after the Big Event. I question whether our concepts of time are meaningful before 'then'.
Well, gravity waves could be a relatively direct probe of the end of inflation. I don't think there's a problem with our concept of time back into the inflationary era, though we obviously have to be wary not to make too strong of statements about what happened before a certain point in inflation.
 
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Chronos
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All things in good time. LISA is up and running. I concede it may have the best chance to pierce the veil. The south pole observatory might also make some waves [or detect them]. We live in interesting times. But, listen to Chalnoth, he is pretty smart. Im a seventies guy, things have changed a lot since then. I knew a lot of really bright guys and gals back in those days. Some of them still publish.
 
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  • #8
Chalnoth
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All things in good time. LISA is up and running. I concede it may have the best chance to pierce the veil. The south pole observatory might also make some waves [or detect them]. We live in interesting times. But, listen to Chalnoth, he is pretty smart. Im a seventies guy, things have changed a lot since then. I knew a lot of really bright guys and gals back in those days. Some of them still publish.
Well, it'll be a little bit yet before the south pole telescope gets its polarimeter (looks like next year, which means no data release until 2011, at the earliest). There will be a number of particularly balloon-borne instruments that will be up and running sooner. They are more-or-less directly competing with the Atacama Cosmology Telescope in Chile, and there are also a large number of balloon-borne instruments.

Note, however, that none of these can directly detect gravity waves. Rather, they hope to be able to detect the signature of gravity waves in the cosmic microwave background through extremely sensitive measurements of the polarization of the CMB. The primary difficulty is that we just don't know how strong this gravity wave signal will be, so we don't know how much sensitivity is actually required to detect it.
 
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The primary difficulty is that we just don't know how strong this gravity wave signal will be, so we don't know how much sensitivity is actually required to detect it.
Asking what do we know? Do we know if there is a 'graviton' or how gravity 'works'? Chronos said he was a seventies guy -- I'm even more dated, a sixties one. So I'm not up to date on the latest here.

I really don't know -- have 'gravity waves' even been confirmed?
 
  • #10
Chalnoth
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Asking what do we know? Do we know if there is a 'graviton' or how gravity 'works'? Chronos said he was a seventies guy -- I'm even more dated, a sixties one. So I'm not up to date on the latest here.
Well, I don't know what you mean by how gravity "works". We know that General Relativity is a fantastically accurate description of gravity, but we also know that the theory is incomplete. From what I hear of theorists in the field, the graviton is nearly certain to exist as a massless, spin-2 particle.

I really don't know -- have 'gravity waves' even been confirmed?
Yes, through observations of the spin down time of pulsars in binary systems: the radio emissions of the pulsar in the system provide a way to measure the period of revolution of the system to extraordinary accuracy. General Relativity makes a specific prediction on how the orbit in such a system will decay, due to the emission of gravity waves. The decay of these orbits matches precisely the prediction of General Relativity.
 
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First I've heard of it -- do you have a reference source I can read?
 
  • #12
Chalnoth
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