Page 1 of 5 123 ... LastLast
Results 1 to 30 of 142

Thread: The Red Shift?

  1. #1
    Join Date
    Feb 2017
    Posts
    71

    The Red Shift?

    The red shift of the galaxies is an empirical fact. Is the expansion of the universe also a fact because of the red shift, or is it an interpretation of this fact in the context of some model, theory? How can the space of the universe expand when in the whole space of the universe there is already all the space that there is?

  2. #2
    Join Date
    Jul 2005
    Location
    Massachusetts, USA
    Posts
    21,422
    The expansion of the universe is not yet an empirical fact. At some point in the distant future when we have parallax distances to galaxies with baselines of a few light years, we may be able to measure the recession velocity of nearby galaxies directly and correlate them to red shifts. When we do that, the expansion of the universe will be an empirical fact. At the moment, it is merely an incredibly successful predictive model. As to how, clearly most human minds aren't well practiced at imagining the whole universe at once, and so language fails to allow a clear easy explanation. Also, there are models, we haven't limited it to one model, so which model would you want explained?
    Forming opinions as we speak

  3. #3
    Join Date
    Jan 2007
    Posts
    1,039
    Quote Originally Posted by Olli S View Post
    The red shift of the galaxies is an empirical fact. Is the expansion of the universe also a fact because of the red shift, or is it an interpretation of this fact in the context of some model, theory? How can the space of the universe expand when in the whole space of the universe there is already all the space that there is?
    The redshift of galaxies is an inference. It's just a really, really good inference. It is one of many things that leads us to a further inference: that the universe is expanding. The redshift of galactic light is based on theories of how materials behave here and in those distant galaxies.

    The entire universe can expand because the geometry of the space itself can change. The average distance between points gets larger over time. But the space within galaxies is far, far from average, so those internal-to-galaxy distances tend not to get larger because gravity holds them in place. (This is a rough way of putting it.)

  4. #4
    Join Date
    Sep 2003
    Posts
    11,488
    Quote Originally Posted by Olli S View Post
    The red shift of the galaxies is an empirical fact.
    To add to the other's comments. The redshifts observed and recorded are emprical facts. Vesto Slipher seems to be the first astronomer to get a number of these from nebulae (galaxies). It took Hubble to later determine that the nebulae were indeed galaxies, though he preferred not to call them galaxies.

    Is the expansion of the universe also a fact because of the red shift, or is it an interpretation of this fact in the context of some model, theory?
    I would think that the redshift as applied to galaxies would constitute a hypothesis, which are always based on objective evidence (ie redshift and other data). The time dilation in the light curves of certain supernovae also contributes objective evidence to the expansion hypothesis, which is part of the broader BBT (Big Bang Theory), of course.

    How can the space of the universe expand when in the whole space of the universe there is already all the space that there is?
    The BBT begins from a tiny size just after a tiny fraction of a second past t=0, with an enormous expansion rate, which is now fairly steady though accelerating slightly, apparently. The question of what it may be expanding into is beyond science since this goes beyond all known observational possibilities.
    Last edited by George; 2017-Feb-13 at 08:47 PM.
    We know time flies, we just can't see its wings.

  5. #5
    Join Date
    Oct 2009
    Location
    a long way away
    Posts
    9,742
    Quote Originally Posted by Olli S View Post
    How can the space of the universe expand when in the whole space of the universe there is already all the space that there is?
    It may be better to think of it as the universe becoming less dense rather than "getting bigger" as that can produce all sorts of misleading ideas.

    And note that expansion was predicted first. Then the redshift-distance relationship was observed. This was consistent with the expansion expected by GR, but not completely convincing. It was the discovery of the CMB that really killed all the other options. Since then even more evidence found that supports the "big bang" (or Lambda-CDM) model.

  6. #6
    Join Date
    Feb 2017
    Posts
    71
    Thank you for fine answers. The expansion is not yet a fact, the redshift is. The other question goes over scientific understanding.

    One relating question more: the universe can be infinite and eternal even in BB, but what is the dense state when "the observable universe expanded from a hot, dense state a finite time ago" (Shaula in the tread "The Red Shift without Expansion of the Space" in ATM 12?.2.2017). How big it was? Was it little, but there was the whole universe? Smaller anyway as now? Or it had no size because the space and time just started then? Was it only the observable universe and not the whole universe?

    What does it mean that the universe is becoming less dense rather than "getting bigger"?

  7. #7
    Join Date
    Jan 2007
    Posts
    1,039
    Given the current estimated size of the universe visible to us, one can extrapolate back at least to a time when this volume was less than the size of a softball and probably much smaller with a reasonable amount of confidence. This is the case even if the universe is infinite in size, since we are discussing the relative size of a given volume now versus then.

  8. #8
    Join Date
    Sep 2003
    Posts
    11,488
    Quote Originally Posted by Olli S View Post
    Thank you for fine answers. The expansion is not yet a fact, the redshift is.
    The expansion can be considered a fact in a general sense because of the strong evidence that supports that view, but it helps to understand the differences between facts and hypotheses. Saying that it is not yet a fact implies that it is on its way to be a fact, which is not what I was suggesting. You can't directly measure expansion of space with your ruler, but you can measure the wavelength shifts with a ruler. When you have multiple lines of evidence coming from different angles pointing to one view, it gives strong support for that view. Black holes are another example since they can not be seen directly but the indirect evidence is very strong. Expansion is one of those as well since General Relativity includes it, redshift supports it, time delay in SN light curves support it, and a ton of other things tied to the Big Bang Theory supports it. The CMBR may be the strongest since it is what made BBT mainstream for most of the strongest critics.

    One relating question more: the universe can be infinite and eternal even in BB,
    BBT has a beginning and it is shortly after t=0 sec. The age of the universe is very close to 13.8 billion years. It was extremely tiny in size and too hot for mass to exist. It immediately began expanding and cooling, allowing mass to essentially condense from the cooling swarm of energy. The expansion simply means that points within this volume got farther and farther apart from each other, like dots on the surface of an inflating balloon, though this example is only two dimensional.
    Last edited by George; 2017-Feb-14 at 04:05 PM.
    We know time flies, we just can't see its wings.

  9. #9
    Join Date
    Sep 2003
    Posts
    11,488
    Quote Originally Posted by Kwalish Kid View Post
    Given the current estimated size of the universe visible to us, one can extrapolate back at least to a time when this volume was less than the size of a softball and probably much smaller with a reasonable amount of confidence.
    I think the softball size refers to the size after the extremely brief and amazing inflationary period.

    This is the case even if the universe is infinite in size, since we are discussing the relative size of a given volume now versus then.
    An infinite universe would not be a reference to the observable universe, which is the only arena where science can work.
    We know time flies, we just can't see its wings.

  10. #10
    Join Date
    Feb 2017
    Posts
    71
    What is the relative size of a given volume now versus then? Less than softball but infinite in size?

  11. #11
    Join Date
    Jul 2005
    Location
    Massachusetts, USA
    Posts
    21,422
    Quote Originally Posted by Olli S View Post
    What is the relative size of a given volume now versus then? Less than softball but infinite in size?
    Olli S, can you please give us some context about what you want to know this for, and how much you already know?
    The problem is that your questions are very open ended, and it would take whole textbooks to give the full range of answers that cover what you might be asking.
    Please help us.
    Forming opinions as we speak

  12. #12
    Join Date
    Mar 2010
    Location
    United Kingdom
    Posts
    6,337
    Quote Originally Posted by Olli S View Post
    One relating question more: the universe can be infinite and eternal even in BB, but what is the dense state when "the observable universe expanded from a hot, dense state a finite time ago" (Shaula in the tread "The Red Shift without Expansion of the Space" in ATM 12?.2.2017). How big it was? Was it little, but there was the whole universe? Smaller anyway as now? Or it had no size because the space and time just started then? Was it only the observable universe and not the whole universe?
    The current theory puts limits on how small the universe can be, but not how large. It is not possible to say if the hot dense state extended throughout the universe or what proportion of the whole universe it represented. Nor is it possible to state that the universe is infinite or finite, bounded or unbounded. Maybe some day we will find a test to distinguish between models with these features, but as of yet we cannot.

    As has been said at the end of the inflationary period models show that the observable universe was 10cm or so across. They say little about any area outside that.

  13. #13
    Join Date
    Mar 2007
    Location
    Falls Church, VA (near Washington, DC)
    Posts
    7,195
    Quote Originally Posted by Olli S View Post
    What is the relative size of a given volume now versus then? Less than softball but infinite in size?
    The radius ratio would be on the order of 1027 to 1, and the volume ratio something like 1081 to 1, if we compare the radius of what we now observe to that of a softball.

  14. #14
    Join Date
    Aug 2008
    Location
    Wellington, New Zealand
    Posts
    3,245
    Quote Originally Posted by Olli S View Post
    Thank you for fine answers. The expansion is not yet a fact, the redshift is.
    I would say that redshifts are observations while an expanding universe is based an overwhelming body of evidence which includes the variation of redshift with distance (Hubble's law).

    In BB the universe can be finite or infinite. The universe is not eternal - its age is 13.7 billion years.
    The "hot dense state" is the state of the entire universe, e.g. if the universe is infinite then all of it is filled with a hot dense plasma. However there is the part of the universe that is visible to us - observable universe. It is the observable universe that is given a size in its early history, e.g. a basketball or softball. or maybe pea.

  15. #15
    Join Date
    Oct 2009
    Location
    a long way away
    Posts
    9,742
    Quote Originally Posted by Reality Check View Post
    The universe is not eternal - its age is 13.7 billion years.
    There are various models in the classical version of the big bang where the universe may be infinitely old. For example, "big bounce" models, eternal inflation, etc.

    And, once we have a theory of quantum gravity, this may change. There is at least one attempt to introduce quantum theory into the big bang model that results in an infinitely old universe (https://phys.org/news/2015-02-big-qu...-universe.html).

  16. #16
    Join Date
    Aug 2008
    Location
    Wellington, New Zealand
    Posts
    3,245
    "big bounce" models generally have problems, eternal inflation is that the inflationary phase lasts forever, No Big Bang? Quantum equation predicts universe has no beginning was promising but this 2014 paper has been only cited 4 times and only by the authors.

    However we do not have to confuse the issue - in standard BB the universe has an age.

  17. #17
    Join Date
    Sep 2003
    Posts
    11,488
    Quote Originally Posted by Strange View Post
    There are various models in the classical version of the big bang where the universe may be infinitely old. For example, "big bounce" models, eternal inflation, etc.
    Did you mean to say there are other theories separate from (and not "versions of") the Big Bang Theory? The BBT is very specific and I'm not aware that it has a bounce version to it, nor an infinite age version as well.
    We know time flies, we just can't see its wings.

  18. #18
    Join Date
    Oct 2009
    Location
    a long way away
    Posts
    9,742
    Quote Originally Posted by George View Post
    Did you mean to say there are other theories separate from (and not "versions of") the Big Bang Theory? The BBT is very specific and I'm not aware that it has a bounce version to it, nor an infinite age version as well.
    These are variations on the same basic model: they all use the same basic description of the universe based on GR, etc.

  19. #19
    Join Date
    Feb 2017
    Posts
    71
    Quote Originally Posted by antoniseb View Post
    Olli S, can you please give us some context about what you want to know this for, and how much you already know?
    The problem is that your questions are very open ended, and it would take whole textbooks to give the full range of answers that cover what you might be asking.
    Please help us.
    I was using the words in the answers that I didn't understand.

  20. #20
    Join Date
    Feb 2017
    Posts
    71

    Points

    There seems to be different views of the BBT. I must read these thoroughly to give exact questions of what I don't understand. I thought that I understand the BB but there seems to be something not clear to me.

    Personally I think we should start from the rationally sound old theory that the whole space of the whole universe is eternal (or no time, no beginning and no end anyway) and infinite (in the sense of limitlessness, no edge, no boundary, no outside, maybe having a radius). How this is with observable universe is not known.

    Science operates with the observable universe, you have said here. Here is one question already: What is the relation between these two different concepts of the universes (which can be the same too?)?

    I will come back with some clarifying questions of the previous when I have understood these now handled.

  21. #21
    Join Date
    Mar 2010
    Location
    United Kingdom
    Posts
    6,337
    Quote Originally Posted by Olli S View Post
    Science operates with the observable universe, you have said here. Here is one question already: What is the relation between these two different concepts of the universes (which can be the same too?)?
    Basically
    Observable universe: What we can see
    Universe: Everything

    The observable universe is likely smaller than the entire universe in current theories. There have been attempts to show that the universe is actually smaller than the observable universe but so far no luck.

    Note that science doesn't bound itself to the observable universe. What has been said is that current models don't give us a way to test if things were different outside the observable universe. That is not to say that future models won't - it may very well be that someone can come up with a model that makes testable predictions that constrain the behaviour or nature of the rest of the universe.

  22. #22
    Join Date
    Mar 2010
    Location
    United Kingdom
    Posts
    6,337
    Quote Originally Posted by Reality Check View Post
    However we do not have to confuse the issue - in standard BB the universe has an age.
    You should probably specify what you mean by age here. The universe has a time for which we can model how it has been evolving. What state it was in before that time we don't have a model for. So we know it has been around for at least 13.8by - but that doesn't mean it was created at that point or that something didn't precede it for an arbitrarily long period of time. The age of the universe in current theory is really just a bound. There is nothing testable that would distinguish between a universe that is 13.8by old and infinitely old at the moment. So long as at the point 13.8by ago it was in a hot, dense state and expanding as required.

  23. #23
    Join Date
    Aug 2003
    Location
    The Wild West
    Posts
    9,140
    Quote Originally Posted by Olli S View Post
    Personally I think we should start from the rationally sound old theory that the whole space of the whole universe is eternal (or no time, no beginning and no end anyway) and infinite (in the sense of limitlessness, no edge, no boundary, no outside, maybe having a radius). How this is with observable universe is not known.
    This sounds like the old steady-state universe model. That model has been shown to be wrong. We have learned that as we look out into the universe and back in time, we see a universe that is different than today's universe. Star formation is still going on today, of course, but the star formation history of the universe peaked several billion years ago.

    We also observe the cosmic microwave background, the CMB. This shows us the state of the universe at about 380,000 years after the "beginning." Essentially, there was only hydrogen and helium gas at that time. All other elements (carbon, oxygen, etc.) were fused during the lifecycle of stars and later recycled into the interstellar medium.

    I say "beginning" because we know something extraordinary happened right around 13.8 billion years ago. We know nothing about before that, or even if there was a "before." But as mentioned, the universe then had to be in a very hot, dense state. And it had to be expanding and cooling quickly. I say it "had to be" because if it wasn't expanding quickly, all the hydrogen would have fused into helium. But it didn't. Hydrogen still makes up 74% of the non-dark matter in the universe.
    Everyone is entitled to his own opinion, but not his own facts.

  24. #24
    Join Date
    Oct 2009
    Location
    a long way away
    Posts
    9,742
    Quote Originally Posted by Olli S View Post
    Personally I think we should start from the rationally sound old theory that the whole space of the whole universe is eternal (or no time, no beginning and no end anyway) and infinite (in the sense of limitlessness, no edge, no boundary, no outside, maybe having a radius).
    That is not "rationally sound", it is just an idea that appeals to you.

  25. #25
    Join Date
    Jul 2005
    Location
    Massachusetts, USA
    Posts
    21,422
    Quote Originally Posted by Olli S View Post
    ... Here is one question already: What is the relation between these two different concepts of the universes (which can be the same too?)? ...
    In your question you use the pronoun "these", but I'm not sure what two different concepts specifically it refers to. Do you mean the observable universe as we see it, and interpreted with a Big Bang model, vs the old Steady State universe? If that is what you're talking about, there is great evidence supporting the idea that the steady state model can't explain everything.

    A few conceptually simple ones are:
    1. Olbers Paradox,
    2. If you attribute red shift to a tired light idea, you can't explain why supernova light curves time-dilate with distance matching what is expected from an expanding universe.
    3. If the universe is infinitely old, where does the new Hydrogen come from?

    These three are not the exhaustive list of supporting evidence, but they are perhaps an easy starting point for you to see why the Steady State Infinite Universe model isn't a possible explanation.
    Forming opinions as we speak

  26. #26
    Join Date
    Oct 2009
    Location
    a long way away
    Posts
    9,742
    Also, there isn't really a good explanation for the cosmic microwave background radiation in Steady State models, whereas it just falls out naturally from the big bang model.

  27. #27
    Join Date
    Aug 2008
    Location
    Wellington, New Zealand
    Posts
    3,245
    Quote Originally Posted by Shaula View Post
    You should probably specify what you mean by age here.
    Age of the universe in standard BB, i.e. the Lambda-CDM concordance model
    In physical cosmology, the age of the universe is the time elapsed since the Big Bang. The current measurement of the age of the universe is 13.7990.021 billion years ((13.7990.021)109 years) within the Lambda-CDM concordance model.[1][2]

  28. #28
    Join Date
    Mar 2010
    Location
    United Kingdom
    Posts
    6,337
    Quote Originally Posted by Reality Check View Post
    Age of the universe in standard BB, i.e. the Lambda-CDM concordance model
    Yes, but the point I was making is that this is not the age from some kind of birth event (the singularity, creation of spacetime or any of the other things that people try to jam into the model). It is the age of the universe measured from the earliest point we can model. There is nothing in the current theory that say that the universe could not have existed before that point. The universe could be infinitely old or have some kind of creation event from which it makes sense to mark the start of the universe, we just don't know. But we choose to define the age of the universe in a way that makes sense within the theoretical framework we have.

    Your statement "However we do not have to confuse the issue - in standard BB the universe has an age." is factually correct only if we define the age of the universe in the aforementioned way. I wanted to make sure that this subtlety was not missed as I think it is important to the questions the OP is asking.

  29. #29
    Join Date
    Aug 2008
    Location
    Wellington, New Zealand
    Posts
    3,245
    Quote Originally Posted by Olli S View Post
    Personally I think we should start from the rationally sound old theory that the whole space of the whole universe is eternal (or no time, no beginning and no end anyway) and infinite (in the sense of limitlessness, no edge, no boundary, no outside, maybe having a radius).
    There is no evidence that the universe is eternal. We have not found any "eternally old" objects. Everything we look at have ages that are less than the age of the universe (time back to the Big Bang in standard cosmology) of 13.7990.021 billion years. We can speculate about what happened before the Big Bang. However it is likely that we will not find any evidence for this since the inflationary period hides the properties of the very early universe from us. Some alterative cosmologies have predictions that might be verified and support an eternal universe.

    There is no clear evidence that the universe is infinite or finite. Fitting CMB data from WMAP and Planck using cosmological models gives that the curvature of the universe includes no curvature. A flat universe suggests an infinite universe. However the results do not rule out a tiny curvature and a possibly finite universe.

  30. #30
    Join Date
    Aug 2008
    Location
    Wellington, New Zealand
    Posts
    3,245
    Quote Originally Posted by Shaula View Post
    It is the age of the universe measured from the earliest point we can model. ...
    Yes: Standard BB (emphasized again) starts from the earliest point we can model (the inflationary period starting around t = 10−32 seconds). Standard BB says nothing about what happens before the BB.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •