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The expanding universe

  1. Feb 7, 2015 #1
    Please forgive my lack of knowledge about the more complex ways that we have determined that the rate at which the universe is expanding is increasing, but one thing puzzles me. I have read that based on measurements of the red shift of light we have concluded that the farther we look out into space we see that objects are moving away from us faster and faster.

    The farther we look the faster it is moving.

    We have determined based on these measurements that the rate at which the universe is expanding is increasing. Why?

    The farther we look out the farther back in time we are seeing. If we know we are looking back in time as we make these observations it seems like the only thing that we could conclude is that these objects move at the rate they did literally billions of years ago.

    If the farther back in time we look we see that these objects are moving faster then why do we not say that these very distant objects moved away from us at a much faster rate in the very distant past?

    How do we know what these objects billions of light years away are doing at the present time at all?
  2. jcsd
  3. Feb 7, 2015 #2


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    We don't know for sure what any of them are doing now but it is fairly absurd to think that the laws of physics didn't hold back then or don't how now. In other words we assume, with a significant degree of confidence, that they are/were doing exactly what we think.

    Also, there's the fact that we see a time-slice of most of the 14 billion years that the universe has existed and there is zero evidence of any such anomalies.

    As for why the expansion of the universe is accelerating, that is unknown. It could be vacuum energy, or it could be Einstein's Cosmological Constant. Since we don't know WHAT it is, we call it "dark energy".

    I suggest the link in my signature. That might clear up some of your issues.
  4. Feb 7, 2015 #3


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    It's not quite correct that the rate of expansion is increasing: it isn't. The rate is decreasing (slowly). But a nearly-constant expansion rate is an accelerating expansion in a very specific sense: objects within the universe are accelerating away from one another.

    If you imagine a constant rate, this is easy to understand: If the expansion is 70km/s/Mpc, then when two objects are 100Mpc apart (a little over 300 million light years), they will on average be moving 7,000 km/s away from one another. When they're 200Mpc apart, they'll be moving 14,000 km/s away from one another.

    If you don't have dark energy, the expansion rate drops fast enough that objects within the universe do not accelerate away from one another: the relative velocity between two objects is always decreasing. The reason why dark energy drives this nearly-constant rate of expansion is that dark energy itself doesn't dilute as the universe expands: the density remains the same no matter how much expansion there is. And since the expansion rate is space-time curvature, which is in turn a result of the matter and energy that makes up the universe, the constant energy density ensures a constant rate of expansion (well, it's not quite constant because there's still normal and dark matter around which do dilute as the universe expands).
  5. Feb 7, 2015 #4
    Thank you for these very good answers.

    I have always felt that it is best to never assume anything and to question everything.

    Looking at this in terms of what objects are doing relative to each other rather than measuring their distances and speed from us makes sense.

    This is a link to one article I read and also what prompted this question. http://m.livescience.com/32260-how-do-scientists-know-the-universe-is-expanding.html

    My big question has always been why I haven't seen much in the way of how time itself is figured into making these measurements. If a galaxy is 5 billion light years away and moving away from us faster than the one that is 1 billion light years away, why we wouldn't we say 5 billion years ago that galaxy moved away from us much faster. The galaxy we see moving slower just 1 billion years ago would mean that as time went on things slowed down.

    Is this article just blatantly wrong? Or is this what we actually observe from our viewpoint?
  6. Feb 8, 2015 #5


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    Well, it is true that the expansion rate has changed over time. We measure how the expansion rate changes over time by getting multiple measurements of redshift vs. distance (e.g. supernova explosions). Once we have enough of these measurements over a large enough range of distances, we can reconstruct the expansion rate as a function of time. This rate of expansion fits a universe where roughly 1/4 of the current energy density is normal and dark matter, while roughly 3/4ths of the current energy density is made up of dark energy.
  7. Feb 8, 2015 #6


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    Chalnoth, phinds, that's not what the OP is asking about.

    The question is: given the observation of faraway objects receding proportionally faster, together with the knowledge that the farther we look the earlier universe we see, what lets us differentiate between 'space is continuously expanding proportional to distance' and 'space was expanding in the early universe but gradually came to a halt'.
  8. Feb 8, 2015 #7


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    That's what I'm saying: we measure it. We measure the expansion rate over time.

    People have also toyed with models where the expansion rate changes with both distance and time, but these generally don't fix observations very well.
  9. Feb 8, 2015 #8
    By measuring over time do you mean that we only take a series of measurements of an object to determine its velocity or do we also take a series of measurements while also taking into account that the light that we collect left the object x number of years ago and the rate at which the object is moving can only reflect what happened x number of years ago. What adjustments are made to our calculations to account for how far back in the past we are looking?
  10. Feb 8, 2015 #9
  11. Feb 8, 2015 #10
    Do we think the Universe is 14 billion years old just because that's as far back as we can see? Is it possible it might be older and we just can't see anything farther away?
  12. Feb 8, 2015 #11


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    If we take the mathematical model which extremely accurately describes the universe after the emission of the CMB (the furthest we can see), and also explains a number of features of the CMB itself, and extrapolate that model back in time, you get a definite beginning.

    Now, the beginning you get is mathematically impossible, so that means that some other theory has to describe the very early universe. The most common is cosmic inflation, though there are some other ideas as well as to what was going on in that very early state.

    But no matter which way you slice it, no matter what theory you have for that very early state, the transition from that early state to the observable universe we see today was still a beginning of sorts. There may well be other things that went on before that time, but our universe began then.
  13. Feb 8, 2015 #12


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    Not at all. One of the best pieces of evidence, in my opinion at least, is that we can't find any stars older than this. For example, the stars that make up a globular cluster can be used to determine its age. The younger the cluster, the greater the proportion of massive, blue stars making up its population. Since these stars burn their fuel and explode as supernova in just a few million years, they won't exist when the cluster is 1 billion years old. (Except a small amount which form through mergers of other stars)

    But high-mass stars aren't the only stars in the cluster. Stars of all types and masses exist. The more massive they are, the shorter they live. So by seeing what the most massive still existing stars are, we can find out how old the cluster is. For example, if the most massive stars we can see are almost exclusively 1 solar mass, then we can say that the cluster has been around for about 10 billion years. (Since it takes about 10 billion years for a 1 solar mass star to burn all its fuel) If the most massive stars are instead 2 solar masses then the cluster would be about 1.5 to 2 billion years old. Any older than this and those 2 solar mass stars would have already burned all their fuel and faded into white dwarfs.

    This is only one way. There are several others, all of which agree that the universe is approximately 13.7 billion years old.
  14. Feb 9, 2015 #13
    Is it true, that the lifetimes of stars are derived from computer simulations?

    Certainly, no human has actually watched any single kind of star from formation to end of MS and clocked the elapsed time
  15. Feb 9, 2015 #14


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    Pretty much.
  16. Feb 9, 2015 #15


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    Well, yes, but computer simulation based on physics that is very strongly supported by empirical evidence. The computer stuff is just a tool. What matters is the theories on which they are built.
  17. Feb 9, 2015 #16
    It seems that the discussion has gone way off topic although very interesting. I would be very appreciative of a straight forward answer to the question at hand which is this

    I want to understand why the farther and farther out we look we observe that objects are moving faster and faster and since the farther we look the farther back in time we are seeing that we don't also conclude that objects would have actually moved faster in the past than they do today?

    What is done in our calculations to account for how far back in the past we are looking?
  18. Feb 9, 2015 #17


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    The observed redshift of an object is often referred to as a measure of "how fast it is moving" away from us, but that's not strictly correct. What the observed redshift actually tells us, directly, is how much the universe has expanded since the object emitted the light we are seeing. Converting this to a "speed of recession" of the object is often done because people seem to think that's easier to visualize, but for the question you're asking, it may be better to think of the redshift as giving the expansion factor.

    In order to obtain a relationship between redshift and distance, we have to have some independent measure of distance. The measure we use is brightness; we have estimates of the absolute brightness of various types of objects (Cepheid variable stars, particular kinds of supernovas, etc.), and we compare these with their apparent brightness to estimate their distance. Then we can look at the relationship between redshift and distance, which is really a relationship between how much the universe has expanded since a given object emitted light we are now seeing, and how bright the object is when we see it. We then compare the observed relationship with the predicted relationship from various different models of how the universe expands. (Converting the apparent brightness to a distance is more for convenience in comparing with models than anything else.)

    A good brief explanation of how this all works is here:

  19. Feb 9, 2015 #18
    Thank you PeterDonis that makes very good sense.
  20. Feb 9, 2015 #19


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    Sort of. Computer simulations are a large part of it, but observations are another major component. Those computer simulations don't just produce the estimated lifetimes of the stars in question, but also a variety of features that the stars should have if the simulations are accurate (e.g. chemical composition and the relationship between mass, temperature, and age). Each observed star should be a valid output of the simulation program given a particular set of inputs. And that's more or less what we see.

    There currently are some unknowns with respect to the most massive stars (because they are the most complicated), but for most stars there's good empirical support for the simulations.
  21. Feb 11, 2015 #20
    Best way to understand this. Someone holds a bat and you sit on a ball in space. ( extremely hypothetical but bare with me.) as the bat hits the ball your sitting ( at rest) the ball will continue to accelerate until it matches the speed of which the bat hit ball with. One of Newtons laws. Now if that ball were to accelerate faster then what force the bat hit it with them the ball would be expanding away from the bat faster then presumed. And now sitting on the ball looking back at the bat, you would say it is getting further away from you. The light we look at us moving and at a fast rate but the universe almost matches that speed. The projected 14.3 billion old universe is only driven off the fact light has traveled that far. And if you think about the ball as the expanding of the universe and the bat as the start of the universe then really the universe can be older because it is acceleration at such a speed light can't keep up with it. And it true because even though dark matter and energy hypothetically exist matter still attracts matter. But even though this matter collectively attracts each other it is still not enough; because other matter is traveling away from each other. When the milky way and andromeda collide in about 2 billion years minus a sig fig or two, we won't even be able to see any other matter because it will be to far from us and light to reach it.
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