What is Expanding universe: Definition and 230 Discussions

The expansion of the universe is the increase in distance between any two given gravitationally unbound parts of the observable universe with time. It is an intrinsic expansion whereby the scale of space itself changes. The universe does not expand "into" anything and does not require space to exist "outside" it. Technically, neither space nor objects in space move. Instead it is the metric governing the size and geometry of spacetime itself that changes in scale. As the spatial part of the universe's spacetime metric increases in scale, objects move apart from one another at ever-increasing speeds. To any observer in the universe, it appears that all of space is expanding while all but the nearest galaxies recede at speeds that are proportional to their distance from the observer – at great enough distances the speeds exceed even the speed of light.As an effect of general relativity, the expansion of the universe is different from the expansions and explosions seen in daily life. It is a property of the universe as a whole rather than a phenomenon that applies just to one part of the universe and, unlike other expansions and explosions, cannot be observed from "outside" of it.
Metric expansion is a key feature of Big Bang cosmology, is modeled mathematically with the Friedmann–Lemaître–Robertson–Walker metric and is a generic property of the universe we inhabit. However, the model is valid only on large scales (roughly the scale of galaxy clusters and above), because gravity binds matter together strongly enough that metric expansion cannot be observed on a smaller scale at this time. As such, the only galaxies receding from one another as a result of metric expansion are those separated by cosmologically relevant scales larger than the length scales associated with the gravitational collapse that are possible in the age of the universe given the matter density and average expansion rate. To paraphrase, the metric is forecasted to eventually begin to outpace the gravity that bodies require to remain bound together, meaning all but the most local bound groups will recede.
According to inflation theory, during the inflationary epoch about 10−32 of a second after the Big Bang, the universe suddenly expanded, and its volume increased by a factor of at least 1078 (an expansion of distance by a factor of at least 1026 in each of the three dimensions), equivalent to expanding an object 1 nanometer (10−9 m, about half the width of a molecule of DNA) in length to one approximately 10.6 light years (about 1017 m or 62 trillion miles) long. A much slower and gradual expansion of space continued after this, until at around 9.8 billion years after the Big Bang (4 billion years ago) it began to gradually expand more quickly, and is still doing so. Physicists have postulated the existence of dark energy, appearing as a cosmological constant in the simplest gravitational models, as a way to explain this late-time acceleration. According to the simplest extrapolation of the currently-favored cosmological model, the Lambda-CDM model, this acceleration becomes more dominant into the future. In June 2016, NASA and ESA scientists reported that the universe was found to be expanding 5% to 9% faster than thought earlier, based on studies using the Hubble Space Telescope.

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  1. K

    I How fast were distant galaxies moving?

    I'm aware that the expanding universe causes galaxies to move away at a velocity proportional to distance. However, I have some interest in the velocity that the galaxy was moving at the time in the past that we are seeing at present. This must be known about because the time dilation of...
  2. S

    I Can the integrated Sachs-Wolfe & the Rees-Sciama effects have any influence on matter?

    CMB photons can be affected by the expansion of the universe through the linear integrated Sachs-Wolfe effect (ISW) [1] and the non-linear ISW effect or also called Rees-Sciama effect [1]. In particular, according to the ISW effect, the photons crossing superclusters would leave them having a...
  3. S

    I Voids, dark energy and tidal forces...?

    I would like to ask you some questions I have about some interesting work I was reading (https://arxiv.org/abs/1205.4238 & https://academic.oup.com/mnras/article/527/4/11962/7457744) where the authors analysed the effects of dark energy in the shape and evolution of voids Apparently, they...
  4. S

    I Photons being blueshifted when crossing evolving voids?

    I was reading this paper (https://academic.oup.com/mnras/article/288/2/387/960778) where they analysed how CMB radiation is affected by evolving voids in an expanding spacetime (particularly through the Rees-Sciama effect and the integrated Sachs-Wolfe effect). This effect predicts that photons...
  5. S

    I Dark energy and the kinematic Sunyaev-Zeldovich effect?

    I was reading this interesting article about possible effects of dark energy in the formation of large-scale structures which should have an impact on the Sunyaev-Zeldovich effect ("Dark energy imprints on the kinematic Sunyaev-Zel'dovich signal" (https://arxiv.org/abs/1309.1163))There, the...
  6. PhanthomJay

    I Hubble Constant, Dark Energy and the Expanding Universe

    Why was the Hubble constant assumed to be decreasing and slowing down (decelerating) the expansion rate of the Universe, while at the same time Dark Energy is presumably accelerating the expansion? And to thicken the plot. recent news from NASA indicates that the Hubble constant is now...
  7. S

    I Dark energy contributing to, or modifying, mass estimates?

    I have found some papers (like this one: https://www.aanda.org/articles/aa/full_html/2009/45/aa12762-09/aa12762-09.html) which say that dark energy increases the potential energy in a system of a quasi-stationary gravitationally bound many-body system. It also says that because of this, the...
  8. S

    I Same state as in the Big Bang in a collapsing universe?

    Suppose the universe were to eventually collapse in a Big Crunch [1]. How closely could the universe's final moments resemble those at the beginning of the universe? Could the universe return to its original state exactly in some kind of "Big Crunch" or "Big Bounce" model? [1]...
  9. S

    I Expansion-Collapse cycle in cosmological structures?

    Concerning cosmological structures (like galaxies, clusters of galaxies, gas bodies, superclusters...etc) if the elements that make them are close enough they will be attracted towards each other by gravity. Contrarily, if they are sufficiently far apart they will get further away following the...
  10. S

    I Avoiding heat death in an accelerated expanding universe?

    https://en.m.wikipedia.org/wiki/Heat_death_of_the_universe Is the heat death of the universe completely unavoidable in an universe with an accelerated expansion dominated by dark energy like ours? Or can there be any way to avoid it according to current knowledge, observations and experiments...
  11. S

    I Methods for energy harvesting in expanding universe?

    There has been much discussion about how could we (theoretically) extract energy from the accelerated expansion of the universe. However, the only gedankenexperiment I can found is the "tethered galaxies" one (e.g. https://arxiv.org/abs/astro-ph/0104349). However, has somebody proposed an...
  12. S

    I Internal energy of a comoving volume increasing as space expands?

    I was reading an article by Edward Harrison, which tackles the problems of conservation of energy at cosmological scales. At some part (point 2.4) he cites several article, including one by Rees and Gott, which he says indicates that the internal energy of a comoving volume (e.g. a cosmic...
  13. S

    I Deviations of conservation laws in cosmological evolution?

    If energy is "not conserved" in General Relativity (or at least, it is difficult to define it) in the context of an expanding accelerating spacetime (like it happens in our Universe), are there any observations of deviations from the strict conservation laws in the evolution and formation of...
  14. S

    I Energy from quantum systems in an expanding universe?

    I found a paper (https://arxiv.org/pdf/astro-ph/0411299.pdf) which talks about quantum systems emitting energy due to spacetime expansion. Is this true or only a hypothesis?
  15. S

    I Particle production in an expanding universe?

    I was reading this interesting article [1] which talks about particle production in an expanding universe. Usually this process is proposed to have occurred in the early universe, when the expansion was in the inflationary phase and it was so powerful that matter was created in particle...
  16. S

    I Matter Gaining Energy from Expanding Spacetime?

    Sean Carroll has an article (https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/) where he explains that matter can gain energy from spacetime expansion. At the end of the article, he says: In general relativity spacetime can give energy to matter, or absorb it from...
  17. A

    I Question about the expanding Universe and the Andromeda Galaxy

    Hi If the universe is expanding, then why does the Andromeda Galaxy approach us? Does not it contradict Hubble's law?
  18. J

    I What is the acceleration of the expansion of the Universe?

    I can't find any values of acceleration or rate of change of acceleration of the expansion of the universe when I looked it up and I need these values for a theory I'm working on that could supersede dark energy and show the universe is closed even if everything accelorating away from us and...
  19. elcaro

    I Energy conservation in an expanding universe

    The total amount of energy is still a conserved quantity, even in an expanding universe based on a positive and constant energy density, and even under the rapid exponential expansion during inflation, total amount of energy is conserved. For how this works, see this lecture by Alan Guth, the...
  20. docnet

    B Speed of Light & Expanding Universe: Does Hubble's Law Violate Rule?

    Hubbles law states the rate of recession of galaxies increases proportionally with distance, and the cosmological horizon is where distant galaxies recede away at the speed of light. Does this not violate the rule of faster than light speed travel?
  21. S

    B Expanding Universe: How Does It Conserve Energy?

    I read somewhere that expanding universes create more energy as they expand, and I was thinking over time there would be a considerable amount of energy created due to this expansion. Even with a very small cosmological constant the energies created over time would probably dwarf anything that...
  22. Arman777

    I Jeans Instability in an Expanding Universe

    I am trying to derive the equation for a case, where we have a dust(zero-pressure) in an expanding universe. There are 4 equations but I think exercising on one of them would be helpful for me. I am trying to derive the equation for a case, where we have a dust(zero-pressure) in an expanding...
  23. Staticboson

    I Does the expanding Universe follow Lorentz contraction?

    As object separate with a receding velocity proportional to the distance, it would seem appropriate to think that objects and space itself, which are located at a distance sufficiently far away (and beyond) to were recession velocities are large enough that Lorentz length contraction effects...
  24. Buzz Bloom

    I Can Lagrangian mechanics be applied to motion in an expanding universe

    Summary: Since L = T - V, and T equals the kinetic energy (KE) of a particle whose trajectory is to be calculated, how is KE defined since some of its motion will be due to the expanding universe? My understanding may well be wrong, but it is the following. if a particle is stationary at...
  25. S

    I CMB Photons: 3000˚ K to 2.7˚ K in an Expanding Universe

    Now...exactly when and how do CMB photons lose their 3000˚ K creation energy to become 2.7˚ K?
  26. CHOP

    I Expanding universe needs a big bang?

    i have read that it is generally thought to be a consequence of the big bang (so, matter in motion) + something (dark energy) making that matter accelerate. why is the big bang needed in it? can't you just have acceleration? by the way, i am not sure what is intended by 'prefix' beside the...
  27. V

    I How can an expanding Universe look homogeneous?

    Observation shows that the Universe is homogeneous (and isotropic) at the large scale, while one expects to see inhomogeneity (increasing density at greater distances) on the past light cone due to expansion. This seems inconsistent. Am I misunderstanding something here?
  28. D

    I The Expanding Universe and a stationary frame of reference

    What is the rate of expansion of the universe and what is it's acceleration rate of expansion? Exactly? I recall it's about 67km/s at 1 mega parsecs? But then what's the acceleration rate? This exact information I cannot find. If the universe is expanding, and this expanding is accelerating...
  29. S

    B Minkowski Diagram: Expanding Universe

    Is this how you would draw the Minkowski space diagram with space expanding, (without acceleration or black holes) ?
  30. S

    B Galaxy Collisions & Expansion: What's Going On?

    Hi, this is just a question I've had for years and have not been able to figure out because of conflicting information. I have read that space is expanding and that it's speeding up, but also have heard that in about 5 billion years our galaxy and Andromeda will collide. How is this possible...
  31. Ennio

    I Expanding universe or tired light in a static universe?

    Has anyone read the book of this author? What is your opinion about this topic? https://www.google.de/search?q=tired+light+lyndon+ashmore&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjbo_exj5vfAhXK2qQKHdHsD6AQ_AUIDygC&biw=1280&bih=623
  32. Z

    B Expanding Universe - Could It Be Rotating?

    Instead of expanding, could the universe be rotating? Everything rotates. Atoms, solar systems, galaxys. Could it be that we just can't see enough of the universe to see it's rotation?
  33. Hugh de Launay

    I Is the increment of the expanding universe part of cosmology

    {Reference: Wikipedia's Friedmann-Lemaitre-Robertson-Walker (FLRW) Metric article)} The FLRW (1935) mathematical model of the universe is the one most used by cosmologists. It is differentiable, which means it is based on sound, consistent, mathematical formulations. (The Lambda-CDM model is...
  34. I

    I Liouville Theorem and the Expanding Universe

    I've a general question. I'm self-studying classical mechanics using various means one of which is Leonard Susskind's Theoretical Minimum lecture series. I'm on Lecture 7 and we are doing Liouville's Theorem. My understanding of it so far is that in phase space as something expands in, say, the...
  35. W

    I Conservation of energy in an expanding universe

    I have read that conservation of energy is not a meaningful concept in an expanding universe cosmology. See here http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/ However I have also heard the if the net energy os the universe is zero then it can have a vacuum genesis...
  36. K

    B Where Does the Energy for the Expanding Universe Come From?

    What is the source of the energy needed by the ever expanding space-time? It could not be baryonic matter, since it accounts for only 4%. It could not be radiation, since it accounts for only 0.008%. Is it dark matter being converted? Is it quantum fluctuations? Kurt Ludwig
  37. platosuniverse

    B Do photons interact with spacetime?

    The way I understand this is that Relativity says space-time is like a field that's affected by the way mass moves through it. Photons are massless so is this why the speed of light is the same in all reference frames?
  38. J

    I Dynamical Casmir Effect in expanding space?

    The Dynamical Casimir Effect is the production of real photons from the vacuum in a system where one has moving mirrors (see https://www.technologyreview.com/s/424111/first-observation-of-the-dynamical-casimir-effect/). The frequency of the photons is related to the ratio of the velocity v of...
  39. E

    I Observational evidence against expanding universe in MNRAS

    << Mentor Note -- after a very long Mentor discussion, we acknowledge that this paper, while potentially controversial, has been published in a reputable peer-reviewed journal. We believe that a discussion of this paper can be useful and constructive. Thanks >> This paper of mine was...
  40. Z

    I Perceived vs. Actual Velocity in an Expanding Universe

    According to what I've been taught, the distance-related redshift seen by Edwin Hubble is an artifact of an expanding Universe. That is, as light travels through space, space itself expands, redshifting the light (matter is embedded in space and does not itself expand but is instead carried...
  41. Orodruin

    Insights Coordinate Dependent Statements in an Expanding Universe - Comments

    Greg Bernhardt submitted a new PF Insights post Coordinate Dependent Statements in an Expanding Universe Continue reading the Original PF Insights Post.
  42. durant35

    I Expanding universe and quantum wavefunctions

    In a dark energy dominated universe, it seems that all the particles get away from each other and that the final state will be one with one or zero particles per horizon. This sounds very intuitive, but it is based on classical physics and GR. Particles have wavefunctions and this is whar...
  43. G

    B How is a Vaccum mathmatically described?

    Anyone know what books and astrophysicist has this knowledge? Thank you, Greg
  44. D

    B How can anyone explain the universal equilibrium density problem

    With NASA defining this equilibrium density at 9.9 x 10<30 gm/cm3, how can an increasingly expanding universe maintain a steady density as it expands?
  45. durant35

    I How will the universe empty out?

    I have a problem understanding where does the matter/radiation go to because of the expansion of our universe. Eventually all normal matter and radiation will leave our causal patch, but the word leave is pretty unconvincing in this case. They will stay in their own causal horizon until each...
  46. durant35

    I Does the number of causal patches increase bc of expansion

    I'm having a hard time understanding some fundamental properties of our expanding universe. It seems reasonable that globally the size of the universe increases bc of the expansion. However the size of each causal patch/observable universe stays the same. Following this logic it would seem that...
  47. hsdrop

    B Is the Expansion of the Universe Uniform?

    I know that the universe is expanding since the beginning of time or at least the beginning of the universe it self. So I was wondering if the universe has always expanded evenly (some parts getting bigger while other grow not as fast) and if it doing so now?? The example in my head is like...