The Edge of the Physical Universe

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Discussion Overview

The discussion revolves around the concept of the observable universe and the limits of our detection capabilities, particularly in relation to the cosmic microwave background and potential observations using neutrinos and gravitational waves. Participants explore theoretical implications, observational limitations, and the nature of time in the context of the early universe.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants suggest that the observable universe's edge at approximately 15 billion years may be a limit of detection rather than a physical boundary.
  • It is noted that the early universe was opaque, and the emission of the cosmic microwave background marks a significant observational limit.
  • Some propose that using neutrinos or gravitational waves could allow us to probe earlier epochs of the universe, potentially revealing information beyond the current observational limits.
  • Concerns are raised about the meaningfulness of time before the inflationary era, with differing views on whether concepts of time apply in that context.
  • Participants discuss the capabilities of current and future observational technologies, such as LISA and various balloon-borne instruments, in detecting gravitational waves and their signatures in the cosmic microwave background.
  • There is uncertainty regarding the existence of gravitons and the completeness of General Relativity as a description of gravity, with some expressing skepticism about the current understanding of gravitational waves.
  • One participant requests references for further reading on gravitational waves and their confirmation through observations of binary pulsars.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the limits of observation, the nature of time, and the existence of gravitational waves and gravitons. The discussion remains unresolved with no consensus reached on these topics.

Contextual Notes

Participants acknowledge limitations in current observational technology and the challenges in detecting gravitational waves, including uncertainties about the strength of signals and the sensitivity required for detection.

Simple10
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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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Simple10 said:
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.
 
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 haven't noticed.
 
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.
 
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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Chronos said:
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.
 
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. I am 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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Chronos said:
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. I am 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.
 
Chalnoth said:
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?
 
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Rymer said:
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.

Rymer said:
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.
 
  • #11
First I've heard of it -- do you have a reference source I can read?
 

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