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  1. #1 Expanding Universe question 
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    I believe Einstein predicted the expansion of the universe would slow down over time which worked with his theories/math...but we now know the universe expansion is accelerating. How can this be? how can the universe's expansion increase? slowing down would make sense due to the attraction of gravity. help me understand please


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    Quote Originally Posted by DaninFlorida View Post
    I believe Einstein predicted the expansion of the universe would slow down over time which worked with his theories/math...but we now know the universe expansion is accelerating. How can this be? how can the universe's expansion increase? slowing down would make sense due to the attraction of gravity. help me understand please
    Your history is not quite right. Here's a much-simplified summary:

    Einstein's GR equations yield different solutions depending on the input data (e.g., total mass of the universe, etc). Einstein himself personally favoured a static universe, as that comported with the sketchy astronomical evidence of the day. Alas, his equations showed that rather unlikely circumstances were required to produce a stable universe; eternal expansion, or a "big crunch" were almost inevitable outcomes. So he added a term to "fix" things up.

    Soon evidence of expansion began to accumulate, and Einstein regretted his modification. Then 20 years ago, Perlmutter et al. discovered that expansion was accelerating, and Einstein's fix was revived. We give the name "dark energy" now to the mechanism. It remains a topic of intense study.

    Your intuition about gravity inevitably slowing things down is wrong. Think of a rocket -- if it travels at escape velocity or faster, it doesn't ever come back.


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    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by DaninFlorida View Post
    I believe Einstein predicted the expansion of the universe would slow down over time which worked with his theories/math...but we now know the universe expansion is accelerating. How can this be? how can the universe's expansion increase? slowing down would make sense due to the attraction of gravity. help me understand please
    Your history is not quite right. Here's a much-simplified summary:

    Einstein's GR equations yield different solutions depending on the input data (e.g., total mass of the universe, etc). Einstein himself personally favoured a static universe, as that comported with the sketchy astronomical evidence of the day. Alas, his equations showed that rather unlikely circumstances were required to produce a stable universe; eternal expansion, or a "big crunch" were almost inevitable outcomes. So he added a term to "fix" things up.

    Soon evidence of expansion began to accumulate, and Einstein regretted his modification. Then 20 years ago, Perlmutter et al. discovered that expansion was accelerating, and Einstein's fix was revived. We give the name "dark energy" now to the mechanism. It remains a topic of intense study.

    Your intuition about gravity inevitably slowing things down is wrong. Think of a rocket -- if it travels at escape velocity or faster, it doesn't ever come back.
    thanks for the reply, a few follow ups...and sorry if i say something incorrect, still trying to understand everything and apologize if using incorrect terms:

    1. gravity slowing down the expansion. logically i would think that if the universe is expanding if you looked at objects on the most far outside, the would have mass behind (interior to them) and essentially nothing in the space that it is expanding into...it would seem then that there would be some type of gravitational pull inward toward that mass that would slow it down over time? it would seem the farthest most object would pull itself inward while the next closest object would be attracted outward to meet it...perhaps resulting in a static universe as you noted einstein believed in.

    2. dark energy - the term you are saying he used to "fix things", are you referring to some type of constant or correction factor? and now we have added that back in after it was deleted but now call it "dark energy"? on the surface that seems odd and a bit awkward? the way you describe it, it sounds like something made up to just make the theory still work?? not trying to be flippant, its an honest question.

    3. the rocket example - the rock is slowing down due to gravity but thats overcome by the engine. dont think that would be correct analogy here to demonstrate your point? if the engine were to stop half way up and remove that force the rocket would continue to slow down while in the influence of gravity and then hit a point where its outside its influence...at that point the speed would remain constant. so back to my initial thoughts, i could see how the universe expansion would either be slowing down, or at constant speed...but speeding up doest seem to add up?
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    Quote Originally Posted by DaninFlorida View Post
    Quote Originally Posted by tk421 View Post
    Quote Originally Posted by DaninFlorida View Post
    I believe Einstein predicted the expansion of the universe would slow down over time which worked with his theories/math...but we now know the universe expansion is accelerating. How can this be? how can the universe's expansion increase? slowing down would make sense due to the attraction of gravity. help me understand please
    Your history is not quite right. Here's a much-simplified summary:

    Einstein's GR equations yield different solutions depending on the input data (e.g., total mass of the universe, etc). Einstein himself personally favoured a static universe, as that comported with the sketchy astronomical evidence of the day. Alas, his equations showed that rather unlikely circumstances were required to produce a stable universe; eternal expansion, or a "big crunch" were almost inevitable outcomes. So he added a term to "fix" things up.

    Soon evidence of expansion began to accumulate, and Einstein regretted his modification. Then 20 years ago, Perlmutter et al. discovered that expansion was accelerating, and Einstein's fix was revived. We give the name "dark energy" now to the mechanism. It remains a topic of intense study.

    Your intuition about gravity inevitably slowing things down is wrong. Think of a rocket -- if it travels at escape velocity or faster, it doesn't ever come back.
    thanks for the reply, a few follow ups...and sorry if i say something incorrect, still trying to understand everything and apologize if using incorrect terms:

    1. gravity slowing down the expansion. logically i would think that if the universe is expanding if you looked at objects on the most far outside, the would have mass behind (interior to them) and essentially nothing in the space that it is expanding into...it would seem then that there would be some type of gravitational pull inward toward that mass that would slow it down over time? it would seem the farthest most object would pull itself inward while the next closest object would be attracted outward to meet it...perhaps resulting in a static universe as you noted einstein believed in.

    2. dark energy - the term you are saying he used to "fix things", are you referring to some type of constant or correction factor? and now we have added that back in after it was deleted but now call it "dark energy"? on the surface that seems odd and a bit awkward? the way you describe it, it sounds like something made up to just make the theory still work?? not trying to be flippant, its an honest question.

    3. the rocket example - the rock is slowing down due to gravity but thats overcome by the engine. dont think that would be correct analogy here to demonstrate your point? if the engine were to stop half way up and remove that force the rocket would continue to slow down while in the influence of gravity and then hit a point where its outside its influence...at that point the speed would remain constant. so back to my initial thoughts, i could see how the universe expansion would either be slowing down, or at constant speed...but speeding up doest seem to add up?
    In a sense I think you are quite right: we observe the expansion to be accelerating, and it is a struggle to develop a theory that can account for this observation. It is cutting edge science, so we do not have all the answers. We have this label "dark energy" but nobody has much idea what it could be.

    Don't forget that in science one seeks to make models of the physical world that account for observations. So there is no shame in adding a "fiddle factor", if that leads to research into why the fiddle factor is needed and what it could physically represent. It's one approach to improving the model.
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    Quote Originally Posted by DaninFlorida View Post
    thanks for the reply, a few follow ups...and sorry if i say something incorrect, still trying to understand everything and apologize if using incorrect terms:
    No apologies needed!

    1. gravity slowing down the expansion. logically i would think that if the universe is expanding if you looked at objects on the most far outside, the would have mass behind (interior to them) and essentially nothing in the space that it is expanding into...it would seem then that there would be some type of gravitational pull inward toward that mass that would slow it down over time? it would seem the farthest most object would pull itself inward while the next closest object would be attracted outward to meet it...perhaps resulting in a static universe as you noted einstein believed in.
    First, I construed your statement to imply that all matter would eventually be pulled back of necessity, so that a "big crunch" was an inevitability. If all you are saying is that gravity keeps pulling, there's no problem with that statement as worded. For objects traveling at or above escape velocity, that pull gets asymptotically weaker, and the velocity asymptotically approaches a constant (=0 for escape velocity exactly), but never reverses sign. Below escape velocity, the object will be pulled back eventually.

    Now, as to your understanding of what causes a static universe, it is more complicated than what you say. Einstein was dismayed that his original formulation for GR would not lead to a static equilibrium. The "fix" I mentioned previously was to add the (in)famous cosmological constant to his equations, but it produced metastable, rather than stable solutions (in effect, it required balancing a marble on the tip of a cone for all time). Discovery of universal expansion killed off the cosmological constant, but accelerating expansion brought it back. The cosmological constant has been reformed as lamda which, when combined with cold, dark matter, produces the standard big bang model ("lambda-CDM" model).

    2. dark energy - the term you are saying he used to "fix things", are you referring to some type of constant or correction factor? and now we have added that back in after it was deleted but now call it "dark energy"? on the surface that seems odd and a bit awkward? the way you describe it, it sounds like something made up to just make the theory still work?? not trying to be flippant, its an honest question.
    Well, that's how science works. You hypothesize something, test it, and then modify or discard it as evidence comes in to support or disconfirm the hypothesis. Only dogmatists refuse to adapt as new information develops. Modifications "just" to make theory work should be viewed as a strength of science, rather than a weakness. Although the modifications initially may seem ad hoc, they survive if experiment and observation comport with the modified theory. In such a case, instead of "ad hoc", we use the term "inspired."

    ETA: cleaned up history of cosmological constant
    Last edited by tk421; March 29th, 2018 at 11:46 AM. Reason: clarified different behaviours at and above escape velocity
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    Quote Originally Posted by DaninFlorida View Post
    I believe Einstein predicted the expansion of the universe would slow down over time which worked with his theories/math...but we now know the universe expansion is accelerating. How can this be? how can the universe's expansion increase? slowing down would make sense due to the attraction of gravity. help me understand please
    Whether or not gravity will cause expansion to slow down or to stop and reverse depends on the density of mass-energy in the universe:

    Source: https://www.physicsoftheuniverse.com...elerating.html

    When it was found that the rate of expansion was accelerating then the simplest explanation is to add an extra energy component (that appears to be associated with empty space) to the equations in order to match what we observe (as tk421 says, that is how science works).

    People are still trying to understand if that is the "correct" description or if some other description would more accurate or more economical.

    We know from quantum theory that there is energy associated with empty space, the problem is that it is many orders of magnitude too big to be dark energy. So one of the challenges is unifying those two models.
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    This seems an interesting news story.

    A newly observed Galaxy with almost no Dark Matter apparently.

    Ghostly galaxy may be missing dark matter - BBC News

    They should rename Dark Matter and Dark Energy so I don't keep getting the two mixed up though.
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    thanks for the replies guys. regarding the addition of dark energy into the equations. Does the addition have any meaning (mathematically) or is it an unitless constant that is being applied to temporarily "fix" the equations to match what we are observing? the way you guys described it, kind of makes it sound like a placeholder while additional information is being researched in order to fully understand its impact. that said, as we learn more, is there a chance that this would lead to another theory and that GR would need to be replaced with something new to describe observations? or would we just apply a limit of GR to a certain limit and the new theory would be used to describe the physics at this further limit of the universe, kind of like how we apply different theories based on the scale of item we are examining?
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    Quote Originally Posted by DaninFlorida View Post
    thanks for the replies guys. regarding the addition of dark energy into the equations. Does the addition have any meaning (mathematically) or is it an unitless constant that is being applied to temporarily "fix" the equations to match what we are observing? the way you guys described it, kind of makes it sound like a placeholder while additional information is being researched in order to fully understand its impact. that said, as we learn more, is there a chance that this would lead to another theory and that GR would need to be replaced with something new to describe observations? or would we just apply a limit of GR to a certain limit and the new theory would be used to describe the physics at this further limit of the universe, kind of like how we apply different theories based on the scale of item we are examining?
    Take a look at the wiki article on the Einstein field equations (EFEs): https://en.wikipedia.org/wiki/Einstein_field_equations

    To answer your question in a meaningful way first requires that you understand something of what general relativity says, and that requires knowledge of what the terms mean in the EFEs. If you aren't familiar with the terms "metric tensor" or "stress-energy tensor" and such, it will be hard to to give you a useful answer without first giving you a primer on GR. But a "pop-sci" summary from Wheeler is that "spacetime tells matter how to move, matter tells spacetime how to curve" (spacetime's fabric constrains trajectories; mass-energy shapes spacetime's fabric). Lambda is part of the relationship between the two (again, take a look at the form of the EFEs).

    In metric units, lambda has units of m^-2 (i.e., per area).
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    Quote Originally Posted by DaninFlorida View Post
    thanks for the replies guys. regarding the addition of dark energy into the equations. Does the addition have any meaning (mathematically) or is it an unitless constant that is being applied to temporarily "fix" the equations to match what we are observing? the way you guys described it, kind of makes it sound like a placeholder while additional information is being researched in order to fully understand its impact. that said, as we learn more, is there a chance that this would lead to another theory and that GR would need to be replaced with something new to describe observations? or would we just apply a limit of GR to a certain limit and the new theory would be used to describe the physics at this further limit of the universe, kind of like how we apply different theories based on the scale of item we are examining?
    As to whether a better theory could come along, the answer in science is always yes. But it is natural and efficient to stay with a working model unless and until that time. In that sense, everything is a placeholder -- the only eternal truth is that there are no eternal truths.

    The reason for experiment is to find out how good the models are, and where they might be deficient. That's the reason for things like the LHC to exist. By going to ever-higher energies (which maps into ever-tinier length scales) we can test our theories over ever-greater ranges. QM has passed all those tests to absurdly high accuracies.

    GR has also been tested extensively. Again, it has passed all of its tests with flying colours. But as in investing, past performance is no guarantee of future returns. It is always possible that a discovery tomorrow will force a complete reformulation of an established theory. Theorists love data that seems suggestive of a problem (as Asimov once said, the most exciting phrase in science isn't "Eureka!" -- it's "that's funny..."), so they're always getting ready for the shift. After all, that's how one wins a Nobel.
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    Quote Originally Posted by tk421 View Post
    Take a look at the wiki article on the Einstein field equations (EFEs): https://en.wikipedia.org/wiki/Einstein_field_equations
    I think John Baez's pages on this are a good overview: The Meaning of Einstein's Equation

    There is some simple maths but you can get a lot out if it just by reading the descriptions.
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    Einstein did not understand that his relativity said anything about gravity. Royal astronomer Eddington pointed it out in a letter to him. Einstein confirmed it. That gave the only way at that time to confirm relativity by direct observation. There was no cosmological constant in it at all. It was a fudge factor introduced years later, and disposed of years after that. The question explained by that fudge factor did not even exist at the time. The observation needing an explanation didn't exist at the time, and the later observation that disposed of the question and the fudge factor didn't exist until years later.

    The question arose when Hubble discovered that the galaxy wasn't the whole universe. The question is why don't the galaxies fall together. It was understood why the moon doesn't fall to Earth, orbits. It was understood why the Earth doesn't fall into the sun, orbits. Earlier it was understood they were mounted on crystal spheres. It was understood why the sun didn't fall into the center of the galaxy, orbits. The flattened disk shape gave it away. But there was no structure in the arrangement of galaxies indicating orbits to keep them from falling together.

    So the astronomers asked Einstein how relativity gravity explains it. He told them it didn't, but if we add a cosmological constant to oppose gravity it would, if not explain, at least balance the math.

    Some years later it was discovered that the galaxies were moving away. Simple and well understood momentum was enough to explain that. No more need for a mysterious, unexplained, constant force opposing gravity. Hoyle disparaged the idea that it implies that it was once all compressed and then expanded. He disparagingly coined the phrase and called the idea a big bang.

    Interpreted as a big bang, and motion through space by momentum opposed by gravity, we expected the expansion to be slowing. The question arises by how much? Over escape velocity or not? Either way it should be slowing. If over it will never stop expanding. If under it will stop, reverse and big crunch. So research began to answer that question. That research unearthed observations are being interpreted as an increasing, not slowing, expansion under a new, mysterious, unexplained, cosmological constant fudge factor.
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    Quote Originally Posted by doitright View Post
    The question arose when Hubble discovered that the galaxy wasn't the whole universe. The question is why don't the galaxies fall together. It was understood why the moon doesn't fall to Earth, orbits. It was understood why the Earth doesn't fall into the sun, orbits. Earlier it was understood they were mounted on crystal spheres. It was understood why the sun didn't fall into the center of the galaxy, orbits. The flattened disk shape gave it away. But there was no structure in the arrangement of galaxies indicating orbits to keep them from falling together.
    Milky Way, Andromeda and Triangulum galaxies as members of our local group are expected to collide or merge in 4 billion years.

    Interpreted as a big bang, and motion through space by momentum opposed by gravity, we expected the expansion to be slowing. The question arises by how much? Over escape velocity or not? Either way it should be slowing. If over it will never stop expanding. If under it will stop, reverse and big crunch. So research began to answer that question. That research unearthed observations are being interpreted as an increasing, not slowing, expansion under a new, mysterious, unexplained, cosmological constant fudge factor.
    Assumed to be dark energy. So Einstein could still have been right.
    HubbleSite - Dark Energy - Did Einstein Predict Dark Energy?
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    Quote Originally Posted by ox View Post

    Assumed to be dark energy. So Einstein could still have been right.
    HubbleSite - Dark Energy - Did Einstein Predict Dark Energy?
    No. Dark energy was invented as a fudge factor. No more meaning than a bookmark has to the information in a book. It shouldn't even be called energy. That implies we understand enough about the data to say energy is causing an acceleration. Byr we don't know there is an acceleration at all. It could be an error in our measurement, or our interpretation. A measurement error could be the result of something about space, time, and motion we don't understand especially when there's so much of it. Just like when we disposed of the fudge factor in relativity when we found out the galaxies are moving away rather than falling together or orbiting. It can be an error of interpretation. Like when we interpreted the sun moving around the Earth because that's what it looks like. We interpret the expansion data as acceleration because that's what it looks like. It may not be acceleration at all.

    We can interpret the data as energy, and study the universe and physics to look for the energy, and we can try to figure out a way to interpret the data in a way that doesn't require energy, and study that interpretation and the physics of the universe and see if that interpretation fits better than mysterious unknown energy.

    If it's energy the people looking for it have a lock on the funding. If it's not energy a guy working alone with his brain could figure out the solution. Then the people with the funding can confirm it Just like they did for relativity.
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