Cosmology FAQ Open Thread

John’s post on light-induced sonic booms has set a bad precedent of actually answering questions. (And it’s been a big hit around the internets, so our server keeps overheating.) Sensing an opportunity, commenters hungry for knowledge have chimed in to ask all sorts of perfectly good questions about cosmology. To keep things on track, let’s divert those questions to this separate thread. So this is the chance to ask all of those questions about the universe you’ve always wondered about. For example:

Q: If I plug in Hubble’s law for the velocity of a galaxy in terms of its distance (v = Hd, where H is the Hubble constant), at large enough distances the velocity will be greater than the speed of light! Doesn’t that violate relativity?

A: Yes, it would be greater than the speed of light, but no, it doesn’t violate relativity. What relativity actually says is that two objects can’t pass by each other at a relative velocity greater than the speed of light. The relative velocity of two distant objects can be whatever it wants. In fact, to be more of a stickler, the relative velocity of two distant objects is completely ill-defined in general relativity; you can only compare velocity vectors of objects at the same point. The notion of “velocity” almost makes sense in cosmology, but you have to keep in mind that it’s only an approximate concept. What’s really going on is that the space between you and the distant galaxy is expanding, which redshifts the photons traveling from there to here, and that reminds you of the Doppler shift, so you (and Professor Hubble, so you’re in good company) interpret it as a velocity. But it’s not a Doppler shift; both you and the galaxy are essentially “stationary” (although that concept is also not precisely defined), it’s just that the space between you is expanding.

In fact I already have a Cosmology FAQ that you’re encouraged to check out, and Ned Wright also has one. But feel free to ask questions here; I’m sure Mark will be happy to answer them.

87 Comments

87 thoughts on “Cosmology FAQ Open Thread”

  1. Sean,
    I found a good introductory reference for my above question about chaotic inflation and differing constants and your possible string landscape answer( You probably saw it on arXiv.org but if not):

    “Eternal inflation and its implications”, by Alan Guth.
    http://arxiv.org/abs/hep-th/0702178

    It’s a great Introductory article.

  2. Sean,

    You said: “I don’t know if it’s likely or not, but it’s certainly promising.” However, the dictionary.com definition of the word promising in the context you are using it is “giving favorable promise; LIKELY to turn out well.”

    Talk of favorite models implies that we really don’t have any evidence guiding us toward a strong “suspect” for the dark matter. By analogy, if when a dectective is asked how likely is it that the murderer will be found and he responds by saying each detective has his favorite suspect, then we would rightly conclude that the police force really has no clue who committed the murder. So, are you saying that we really don’t have any clue what the dark matter is? and, if so, how can it be “certainly promising?”

  3. There was an article in physorg today, with the lead… “A hidden twist in the black hole information paradox”.

    It ends with, “Quantum information cannot be completely hidden in correlations: Implications for the black-hole information paradox” appears in the latest Physical Review Letters.

    Sorry for being so dumb on this, but just what does ‘information’ mean in this context?

  4. hi,
    can we discuss about gravity wave background, why should there be one and how is it related to inflation ?
    and can it be seen ?

  5. Dark Matter and Dark Energy seem to be “in these days”. If there is interest in another possible candidate for Dark Matter, read comment 30, in Sean’s “find of Dark Matter and Sterile neutrinos” February 10, 2007 blog. If not turned-off by what is shown and interested in “the rest of the story” (even for Dark Energy) more can be said!

  6. hello guys, no I am disapppointed as no one has answered my ques. Come on, I want to know why should there should be a GWBR and how is that connected to inflation. Sean please! I will be looking forward to ur answer!

  7. Sean, I have brought this up before . maybe you and other readers of CV
    can chime in again.
    My queston is what exactly did
    Kopeikin-Fomalant experiment measure?
    Sergei insists that his measurement is a model-independent measurement of
    speed of gravity and has provided (IMO) reasonable counterarguments
    to his critics.
    However even the papers which critiqued his measurement take completely
    diffrenet point of view. See Samuel’s paper which says that this does not
    even measure speed of gravity in GR.
    What do readers of CV think?

  8. zeenia– There are different possible sources for a gravity-wave background, and searching for it is an ongoing project. Any major ruckus in the early universe is a candidate to make gravitational waves; for example, a strong phase transition (analogous to bubbles in boiling water) could stir things up and leave gravitational waves behind. Even better, gravitational waves could be created during inflation, just as density perturbations are. People are trying to detect the influence of such waves in the cosmic microwave background, and perhaps someday via a dedicated satellite.

    Shantanu– Measuring the “speed of gravity” is like measuring the “speed of blue”; the concept just doesn’t apply. You can talk about the speed of propagation of gravitational radiation — in that case, the prediction of GR is unambiguous that gravitational waves propagate at the speed of light. Arguing about whether some particular measurement is really probing the “speed of gravity” is a waste of time; the important thing is that no observation yet performed is inconsistent with the predictions of GR.

  9. Sean,

    I am not sure if the study of the CMBR is your area of cosmological expertise, but I was wondering, what is your opinion of the conclusions reached in a recent paper titled “On the apparent lack of power in the CMB anisotropy at large angular scales” by Amir Hajian of Princeton University? (astro-ph/0702723) When the COBE data was released one of the puzzling unexpected aspects of the full-sky CMB maps was the absence of long wavelength fluctuations; this odd feature, odd in the sense that it is at odds with inflationary theory, also exists in the WMAP data (or does it?). In other words, if the Universe is infinite and homogeneous, as inflation predicts, it is awfully improbable that we should not be seeing long wavelengths in our tiny portion of the cosmos.

    But now, as reported in the above mentioned paper, it appears that there is no deficit in large scale power in the full sky maps, and the explanation for the apparent lack of large scale power is that masking the sky near the galactic plane creates this appearance.

  10. Pingback: Catholic Priest Proposes New Model for Creation | Cosmic Variance

  11. Is this thing still on?

    If so, I have a question.
    Why can’t physicists use the standard model to calculate exact decay constants for radionuclides, thus alleviating us geologists from having to measure them?

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