What is Accelerating universe: Definition and 43 Discussions

Observations show that the expansion of the universe is accelerating, such that the velocity at which a distant galaxy recedes from the observer is continuously increasing with time.The accelerated expansion was discovered during 1998, by two independent projects, the Supernova Cosmology Project and the High-Z Supernova Search Team, which both used distant type Ia supernovae to measure the acceleration. The idea was that as type Ia supernovae have almost the same intrinsic brightness (a standard candle), and since objects that are further away appear dimmer, we can use the observed brightness of these supernovae to measure the distance to them. The distance can then be compared to the supernovae's cosmological redshift, which measures how much the universe has expanded since the supernova occurred. The unexpected result was that objects in the universe are moving away from one another at an accelerated rate. Cosmologists at the time expected that recession velocity would always be decelerating, due to the gravitational attraction of the matter in the universe. Three members of these two groups have subsequently been awarded Nobel Prizes for their discovery. Confirmatory evidence has been found in baryon acoustic oscillations, and in analyses of the clustering of galaxies.
The accelerated expansion of the universe is thought to have begun since the universe entered its dark-energy-dominated era roughly 4 billion years ago.
Within the framework of general relativity, an accelerated expansion can be accounted for by a positive value of the cosmological constant Λ, equivalent to the presence of a positive vacuum energy, dubbed "dark energy". While there are alternative possible explanations, the description assuming dark energy (positive Λ) is used in the current standard model of cosmology, which also includes cold dark matter (CDM) and is known as the Lambda-CDM model.

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  1. P

    I Energy of Light in Accelerating Universe: Comparing Cases

    Assume two observers very far from each other, so far, that the accelerating expansion of the universe matters. (edit: But not outside of each others event horizons.) They will send light beams each other, and measure the energy of it. Also tie them together with a very long rope to fix their...
  2. S

    I Receding movement of a stopped object in an accelerating universe?

    Imagine we attach an imaginary cosmological scale rope to an object that is very far away from us. Before attaching the string, the object would be receding from us due to spacetime expansion. After attaching it, tension would form in the string and we would eventually stop the object. After...
  3. R

    A Hubble Tension and Cosmic Acceleration: A measurement artifact?

    By analyzing 91,742 reported extra-galactic distances and their one sigma uncertainties for 14,560 galaxies, it was found that pairs of reported extra-galactic distances of the same galaxy differ from each other by 2.07 the reported uncertainties on average. In my opinion, this indicates that...
  4. R0dr1go

    B Acceleration of time due to entropy

    Hello, I am a 15 year old who has done research around the topic of why the universe is expanding at an accelerating rate, and how it will come to an end. After years of thought (since I was 11) I have come up with my hypothesis that time itself is accelerating and slowing down, and has been...
  5. Ranku

    I Accelerating Universe and spherical distribution of matter

    If we take a spherical distribution of matter wherein gravitational force and cosmological-constant force are equal upon an object on its surface, then does the time that it took for that volume to grow to the size wherein the two forces are equal match the time it took for the universe to start...
  6. Bosko

    Stargazing Accelerating universe - luminosity data

    I am looking for the measurement data for distant objects (e.g. type Ia supernovae) The red shift and the luminosity flux data. What time interval is used in the luminosity flux measurement? What formula is used for the luminosity distance calculation?
  7. R

    I Accelerating universe explained by constant expansion?

    Why is the accelerating expansion of the universe not explained simply by the perspective of acceleration caused by a constant (non accelerating) universal expansion? Let's take two particles (X,Y) in a 1d expanding universe where it expands universally by 1 unit per per unit time. O represents...
  8. Gort

    A Observational evidence for accelerating universe

    This question was triggered by the fact that Adam Riess is making his lecture rounds at a local University. So, wanting to be prepared, I pulled out his old Nobel lecture, which nicely described the techniques used for the high-z s/n measurements. I was particularly interested in how they...
  9. H

    Question: Accelerating Universe = Net Positive Energy?

    Physicists have observed the cosmic radiation background to conclude that the universe is flat (or within the margin of error of being flat). This means that the Universe contains the critical density needed to keep it flat, which is a mix of ordinary matter, dark matter and dark energy (I...
  10. Jaami M.

    I The Universe accelerating and light

    How are we, and the universe effected from its accelerating expansion? Also how will our perception of the world around us be effected, since the universe is expanding and light is constant? <-Will things appear to move slower? I know that we only experience just a small fraction since the...
  11. endless

    B Questions regarding to the Accelerating universe from Novice

    The Universe is constantly expanding, and from the observation of Hubble, super novas, the figure turns out to be a acceleration. The theory that's most accepted seems to be dark energy at work. As the story goes, before big bang, there was nothing, maybe not even time or space, so how does...
  12. V

    Accelerating universe question

    Can we really say that the universe is accelerating its expansion because we find that galaxies further away have more redshift? We know that we are looking back in time when we look at the stars, and when we look at the galaxies furthest away from us, we are seeing them as they were a long time...
  13. SpiderET

    Acceleration of expansion of Universe not that fast?

    There is some surprising twist regarding measurements of accelerating expansion of Universe. Maybe it is not accelerating that fast as previously predicted. It seems that there is something questionable about Ia supernovae. What do you think about it? Is this real game changer or some minor...
  14. Damo ET

    Understanding the accelerating universe.

    For the record, I completely accept the concept of an accelerating universe and the role dark energy plays (as the guys who figured these ideas out are light years beyond what I can hope to wrap my brain around). So, I am hoping someone can explain to me in layman terms why this the case...
  15. B

    Is an accelerating universe consistent with the conservation of energy

    Hi, Is the fact that the expansion of the universe accelerates consistent with the conservation of energy? If so, how? Grateful for response!
  16. A

    Measuring Redshift to Support Accelerating Universe

    Universe appears to be expanding at an increasing rate. What measurements were performed to support this idea? My current understanding is that in general redshift of distant stars/galaxies is increasing with time. So to me the measurements may look like this: We choose one star...
  17. B

    Accelerating Universe: Stephen Hawking, Carl Sagan, Arthur C. Clarke

    Hi, first post on the forum :smile: I am watching this video of a conversation with Stephen Hawking, Carl Sagan and Arthur C. Clarke and about 9 minutes into the film Carl Sagan says: "The galaxys are running away from each other. The further away they are from each other, the faster they are...
  18. D

    Accelerating universe question

    It is well accepted that the universe is expanding at an increasing rate. Looking at the evidence, I certainly accept the universe is expanding as well, but is it expanding at an increasing rate? I also understand it as an expansion of fabric of space, no simply a trait of motion between...
  19. S

    Questions about the accelerating universe

    Since the most distant galaxies appear to be faster than the speed of light, due to the insertion of new space, does this mean that galaxies haven't actually gained any velocity as the universe accelerates?
  20. J

    Particle creation in an accelerating Universe?

    Hi, I am interested to hear what people think of the following argument that in an accelerating Universe virtual particles that are separated by the Hubble radius R(t) or more cannot annihilate and thus become real particles. Thus, by the uncertainty principle, particle pairs with energy E...
  21. J

    Constant Hubble parameter -> accelerating Universe?

    Hi, Let us assume that Hubble's Constant H is really constant. Therefore: a' / a = H where a is the scale factor. The solution to this equation is: a(t) = exp(H t) This equation describes an accelerating universe with deceleration parameter q given by: q = - a'' a / a'^2 =...
  22. S

    Accelerating Universe Expansion’s Implications? (layman’s questions)

    I’m new to these forums and as a matter of personal curiosity, and as someone who can’t follow the math behind the basic concepts, I have a few questions. The first set have to do with the accelerating expansion of our universe. A) Does this acceleration affect the inflationary model of the...
  23. A

    Universe Expanding: Is Gravity a Consequence?

    As the universe expands... Is it that we are more on the surface of a balloon, so the expansion is similar for us as it is for our most distant material counterparts? If this is the case than is gravity a consequence of matter resisting time (since the ?big bang or something?) As we...
  24. G

    Accelerating universe - details and dispute

    "accelerating" universe - details and dispute I recently sent an email to several physics professors at CalTech and MIT with a question I have about Cosmology and only one professor (Walter Lewin) replied simply saying good luck! Its not so much of a question as it is a problem. I've talked to...
  25. e2m2a

    Mach's Principle and the Accelerating Universe

    With the current astronomical data indicating the universe is expanding at an accelerated rate, does this not invalidate the Mach's Principle? My understanding is when Einstein tried to incorporate this principle into his general theory, he assumed the universe had to be bounded and closed...
  26. O

    Flat and accelerating Universe

    Hi! I'm doing physics project and I'm stuck with a really awful paradox.:confused: Universe appear to be flat (nearly), so it should expand forever (the critical universe). On the other hand we've got acceleration of the universe so open model. Which model is the right one?
  27. marcus

    Entropic Accelerating Universe (Smoot et al)

    I just posted this on the bibliography links thread. Thought it would be good to post for discussion as well. http://arxiv.org/abs/1002.4278 Entropic Accelerating Universe Damien A. Easson, Paul H. Frampton, George F. Smoot 10 pages, 1 figure (Submitted on 23 Feb 2010) "To accommodate the...
  28. O

    Accelerating Universe, Relativistic Mass and Dark Energy

    Hi! Well, I have a problem, I'm writing Extended Project For Applied Science about Death of the Universe, I am stuck. I am really desperate so I decided to put it on forum. First: The Universe is accelerating so, according to Lorentz's Factor, its mass should be increasing m = \gamma m_{0} =...
  29. M

    Accelerating universe alternative

    I read an article about 2 weeks ago (another astronomy major sent it to me, but unfortunately I'm having trouble find it, when I talk to him again I'll get the source and post it) that proposed an alternative to the universe expanding. The article stated that the reason we see light redshifted...
  30. Ranku

    Accelerating universe, decreasing hubble constant

    I wanted to know a bit more about the fact that in the presently accelerating expansion of the universe the Hubble constant is still decreasing. When the universe was decelerating the Hubble constant was decreasing. It is still decreasing in an accelerating universe. Does that mean the Hubble...
  31. C

    Accelerating Universe: Rate of Expansion Constant?

    is the rate of expansion of the universe constant? or is accelerating or decelerating? can anything such as light ever catch the limits of the universe? and is it the space and the matter of the universe that's expanding or is it just the matter that's separating farther and farther apart into...
  32. S

    Accelerating Universe: Value of Acceleration?

    What is the value of acceleration of universe? like Earth's gravitational acceleration is around 9.8 m/s2..has anyone found this value?
  33. S

    Type1a supernovae as evidence for an accelerating universe

    hi i just wanted to ask if someone could explain this graph for me http://hyperphysics.phy-astr.gsu.edu/hbase/astro/univacc.html thank you
  34. D

    Accelerating universe = conservation of energy?

    I'm sorry if this sounds as a very simplistic view but my main underlying belief that the universe is not accelerating and will not expand to infinity but indeed collapse on itself eventually arises from the fact that the kinetic energy of the expansion will be met by the gravitational...
  35. marcus

    Dark Energy and the Accelerating Universe (survey)

    http://arxiv.org/abs/0803.0982 Dark Energy and the Accelerating Universe Joshua Frieman (Chicago/Fermilab), Michael Turner (Chicago), Dragan Huterer (Michigan) Invited review for Annual Reviews of Astronomy and Astrophysics; 53 pages, 18 figures (Submitted on 7 Mar 2008) "The discovery...
  36. B

    Accelerating Universe: Understanding Deceleration Parameter & Implications

    We know through observations of distant SN that the deceleration parameter is negative and this implies that the universe is accelerating. This is just me but when I think about acceleration, I think of a vector quantity with units of m/s^2 and has magnitude and direction. do we know the...
  37. D

    Accelerating Universe: How Is It Possible?

    This site: http://curious.astro.cornell.edu/question.php?number=575 [Broken] , states that "Astronomers now have strong evidence that we live in an "accelerating universe" how is that possible, where does the repulsion force come from?
  38. A

    Accelerating Universe pre-1997

    I'm a physics grad student attempting to do a research paper on the social implications of the accelerating universe. To do this, I'm trying to find information on theories for and against the accelerating universe from before the supernova observations by the Perlmutter and Riess teams (which...
  39. S

    Why do scientists believe the expansion of the universe is accelerating?

    I have heard that people say the universe is accelerating, however i cannot find direct evidence of these claims anywhere. What has led people to believe that the expansion of the universe is accelerating?
  40. C

    Accelerating Universe: Experiments, Acceptance & Implications

    I've heard that in the late '90s two independent groups discovered experimentally that the expansion of the universe is in fact accelerating, which might imply a non-vanishing cosmological constant in Einstein's relativity. So first of all, can anyone provide some more information about these...
  41. M

    Accelerating Universe and the Loss of Mass

    I had a thought the other day that I would quite like feedback on: Could the universes accelerating expansion be fuelled by the amount of mass in the universe decreasing? Is it possible that a black hole could become so massive that it tears away from/gets pinched off spacetime, how much mass...
  42. M

    Unraveling the Mysteries of the Accelerating Universe

    As the universe expands and disperses, there is less of a gravitational well for photons to climb out of before they reach us. Photons from the early universe must climb out of the gravity well of a more densely populated universe than photons emitted from sources today. Therefore early photons...
  43. P

    Exploring the Possibilities of an Accelerating Universe

    What if... Gravity is the consecuence of mass deforming space-time. But why mass deforms space-time? What if gravity is the consecuence of mass inertia in an accelerated universe. I can imagine a universe expanding at an accelerated speed that is being deformed by the inertia of the objects...