What is Big bang: Definition and 910 Discussions

The Big Bang theory is the prevailing cosmological model explaining the existence of the observable universe from the earliest known periods through its subsequent large-scale evolution. The model describes how the universe expanded from an initial state of high density and temperature, and offers a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements, the cosmic microwave background (CMB) radiation, and large-scale structure.
Crucially, the theory is compatible with Hubble–Lemaître law — the observation that the farther away galaxies are, the faster they are moving away from Earth. Extrapolating this cosmic expansion backwards in time using the known laws of physics, the theory describes an increasingly concentrated cosmos preceded by a singularity in which space and time lose meaning (typically named "the Big Bang singularity"). Detailed measurements of the expansion rate of the universe place the Big Bang singularity at around 13.8 billion years ago, which is thus considered the age of the universe.After its initial expansion, an event that is by itself often called "the Big Bang", the universe cooled sufficiently to allow the formation of subatomic particles, and later atoms. Giant clouds of these primordial elements – mostly hydrogen, with some helium and lithium – later coalesced through gravity, forming early stars and galaxies, the descendants of which are visible today. Besides these primordial building materials, astronomers observe the gravitational effects of an unknown dark matter surrounding galaxies. Most of the gravitational potential in the universe seems to be in this form, and the Big Bang theory and various observations indicate that this excess gravitational potential is not created by baryonic matter, such as normal atoms. Measurements of the redshifts of supernovae indicate that the expansion of the universe is accelerating, an observation attributed to dark energy's existence.Georges Lemaître first noted in 1927 that an expanding universe could be traced back in time to an originating single point, which he called the "primeval atom". Edwin Hubble confirmed through analysis of galactic redshifts in 1929 that galaxies are indeed drifting apart; this is important observational evidence for an expanding universe. For several decades, the scientific community was divided between supporters of the Big Bang and the rival steady-state model which both offered explanations for the observed expansion, but the steady-state model stipulated an eternal universe in contrast to the Big Bang's finite age. In 1964, the CMB was discovered, which convinced many cosmologists that the steady-state theory was falsified, since, unlike the steady-state theory, the hot Big Bang predicted a uniform background radiation throughout the universe caused by the high temperatures and densities in the distant past. A wide range of empirical evidence strongly favors the Big Bang, which is now essentially universally accepted.

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  1. SilverMain

    I Questions about the Location of the Big Bang and the Age of the Universe

    Assumed facts: Galaxies are traveling at velocities that cause very little compression of space. The big bang occurred at one point in space and all matter is moving away from this location. The universe is continuing to expand The farthest galaxies are about 92 billion light years apart or 46...
  2. E

    A Dimensions: Emergent from the Big Bang?

    If we accept the concept that the universe began as a zero-dimensional point, wouldn't that imply that our 3+1 dimensions, or 9+1 / 10+1 dimensions of string theory, emerged at the moment of the Big Bang or some presumably Planck-order time after? Are there any theories on this?
  3. M

    I Big bang occuring everywhere vs everything "compressed"

    Hi I am trying to reconcile the concept of the Big Bang of occurring "everywhere" (an infinite Universe) - v - the concept of the origin of the Big Bang occurring in a particular compressed state and then expanding? Thanks Martyn Arthur
  4. 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]...
  5. J

    I The Big Bang as a White hole

    *********Disclaimer******** Hi, I want to ask this community about some black hole shinanigans. I've spent some time searching for this topic here because I don't want to be the guy who spams a forum with a question already answered a hundred times over. Since none of the threads I found...
  6. D

    I Could it be that the Big Bang had no cause?

    Everything that I’ve researched into this seems to suggest that we know absolutely nothing about the universe at the beginning of the Big Bang event. Could it be that just nothing is there? Could everything in the universe be described by cause and effect, yet cause and effect does not apply...
  7. S

    I No symmetries in the Universe at the Big Bang...?

    I apologize in advance if this is a stupid question but... According to some scenarios about the beginning of the universe (namely cosmological inflation), in layman's terms, everything was born out of a quantum fluctuation which caused a violent expansion. In this case, since an expanding...
  8. Sumeru

    Please express this theory of mine mathematically or through equations

    Can you please express this theory mathematically or through equations? Destiny is the automatically predestined chain or course of events spread throughout the universe that originated from the first cosmic event called the Big Bang. It automatically determined when, where and exactly what...
  9. gurbir_s

    A Solving this first-order differential equation for neutron abundance

    The time rate of change of neutron abundance ##X_n## is given by $$\frac{dX_n}{dt} = \lambda - (\lambda + \hat\lambda)X_n$$ where ##\lambda## is neutron production rate per proton and ##\hat\lambda## is neutron destruction rate per neutron. Given the values of ##\lambda## and ##\hat\lambda## at...
  10. H

    B Does Roger Penrose's Big Bang cyclic-universe thing make sense?

    Does it make sense to anyone? He says our universe is just one in a long line of prior universes. But the big bang was not so much starting from a very small area, but instead results out of a prior universe that had gotten so old that essentially it has no matter, and when there is no matter...
  11. crakedhead

    B Why the expansion after the "Big Bang" is showed in cylindrical form?

    Everywhere on the net, there is no image that the expansion has a departing point, through an expansion in spherical form... Why is that?
  12. H

    B Size of universe - rewinding big bang versus "flat" universe

    I understand, given dark energy, given the current expansion of the universe, given its ~13.8 billion year old age, we calculate the universe is like 93 billion light years across currently. Something like that. But I was listening to something the other day, and when they test the curvature...
  13. H

    B Why did the Big Bang happen?

    Why did it happen? What force would make the universe, which is contracted into a very, very small space, blow outward and expand? Another random question: I've heard that, at least in the inflationary model, the universe started in an amount of space like the size of an atom, or less. How...
  14. J

    I Question about the big bang singularity

    In big bang theories where our universe begins from a singularity (ignoring other theories just now), what would happen if the universe didn't begin as a single point but rather began as energy in an uneven shape? Would we be able to tell from the way the universe is now? Do we know that if the...
  15. T

    B Expansion of Space Itself, rather than due to an Explosion like the Big Bang?

    Please explain how we know that it is space itself that is expanding rather than an expansion due to an explosion such as the big bang. In either case wouldn’t every object appear to be moving away from us? How do we determine that space/ time is expanding? Tex
  16. Cody Livengood

    B What if we cut out the Big Bang and inflation?

    Are the Big Bang and inflation really needed to account for the large-scale structure of the universe, nucleosynthesis, baryon acoustic oscillations, and the anisotropies of the CMB? Can the universe not begin in an already expanded state and still have all those things happen just the same...
  17. SH2372 General Relativity - Lecture 9

    SH2372 General Relativity - Lecture 9

    0:00 Gravitational lensing 23:19 Cosmology 33:00 Robertson-Walker spacetime 38:20 FLRW universe 1:01:35 Big Bang
  18. G

    I Paradox: How can we look back at the Big Bang?

    The Webb telescope images show galaxies that are so far away that it takes ages for their light to reach us, so long that we can see galaxies as they were soon after the big bang. How amazing - but hold on. If we came from the big bang, and nothing travels faster than light, how come we are here...
  19. Physics Slayer

    B Exploring Negative Energy at the Big Bang: A Non-Mathematical Explanation

    I was reading "Brief answers to big questions" By Hawking, the above pic is from a page of the book, it says that at the time of the big bang there was an equal amount of positive and negative energy, and that the negative energy never went anywhere, the space-time itself is a store of negative...
  20. C

    B Is Entropy the Key to Understanding the Big Bang and the Fate of the Universe?

    I am a software trainer and know about as much Physics as I've been able to pick up from the BBC‘s occasional documentaries. I have never even taken a physics course in high school or college so I am an absolute lay-person here with what’s probably a very lay-person question… I was watching...
  21. M

    I The Big Bang and the original size of Universe

    I am trying to understand / visualise, recognising that the Big Bang is not thought of as an explosion, how the Universe could have come into existence as a 'point' but came into existence 'everywhere'? Is it a concept that such a point was then everywhere, and that the mass of the Universe, as...
  22. M

    B Big Bang Question -- How was the first matter formed?

    Big Bang singularity can never be solved, so Could the "big bang" have been an event where a large sum of highly-dense dark energy converted into mass, and in doing the result is like a nuclear explosion? Could the "big bang" just have been a large sum of matter where the core becomes super...
  23. S

    I Entropy after the Big Bang

    If the universe was very hot right after the Big Bang how come the entropy of the universe was lower at that point than now? Isn't heat a reason for higher entropy?
  24. J

    B Basic confusion about the Big Bang and light cones

    I’m getting confused somewhere, and I’d be obliged if someone could pinpoint my error. 1. At or near the Big Bang, everything was so close as to be within each other’s light cones. 2. All parts of the cosmos are now outside of some other parts’ light cones. Therefore, 3. Something traveled...
  25. K

    B Big Bang: Size of the Universe at Different Epochs

    Dear PF Forum, I've been searching what was the size of the early univese, but I think I might have hit the wall. I read that in the end of inflationary epoch, 10-32 seconds, the size of the universe was as big as a grape fruit? And after that, it seems that I can't find a reference to the size...
  26. gregatphysics

    I Do all geodesics begin and end at the big bang?

    So. It was late night, the limpid sky a near cloudless darkness, somewhat lightened by the waxing moon. I being somewhat stoned and looking at the stars and constellations as I sometimes do, and it came to seem to me, in my imagination, that In the spaces between the stars, I was observing (in...
  27. two OLD 2 care

    I Why is the Big Bang shown always directional, instead of globally?

    I`m new to physics, but old to the plant ( 70 ) My question is 1) why wouldn`t the universe have evolved , ( change from little / big ), leaving a bit of both , i.e. 2 theories ? 2) Why is the Big Bang always depicted, directionally ?
  28. Tertius

    A Higgs field was zero right after Big Bang, could it return to zero?

    Quote from cern: "Just after the big bang, the Higgs field was zero, but as the universe cooled and the temperature fell below a critical value, the field grew spontaneously so that any particle interacting with it acquired a mass." Can it go back to zero? If anyone has a comment either way...
  29. 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...
  30. S

    A Any model or theory in physics akin to "Law without law"?

    When trying to explain from where did all the laws come from, John Wheeler proposed the anaphorism of "Law without law". He proposed that at the "beginning" there were no laws whatsoever, only pure chaos, and that they emerged from randomness and chaos when our universe was created. In his own...
  31. C

    I What atoms are we made of, how much is from the big bang?

    I was thinking about deuterium and that it is not made in stars (too reactive/unstable) and not made in super nova as far as I understand, so I am presuming all of it was made in the first few minutes of the universe and has been with us ever since as it comes from no-where else I am aware of...
  32. elcaro

    I Is it time to take the 'bang' out of the Big Bang?

    Paul Steinhardt: Time to Take the ‘Big Bang’ out of the Big Bang Theory? Paul Steinhard, who co-founded inflationary cosmology, is working on alternatives for inflation in the form of a cyclic, bouncing universe model. In this lecture he compares his model with the inflationary model, and...
  33. G

    B Can particles appear from "actual" nothing including no space?

    Hi and thank you for answering questions on complex subjects like this. I’m a non-scientist who is jealous of your brain capacity! Recently, in a discussion with a chemist, he said the particles appearing from nothing might have been the initial cause of the universe. I’m familiar with the...
  34. Physics4Funn

    A Exploring Metrics Before the Big Bang: What Does Zero Mean?

    I know about these metrics Euclidean metric Einstein manifold Riemannian, pseudo-Riemannian manifolds or Riemannian space Lorentzian manifold Minkowski space Kähler manifold Schwarzschild metric Kerr, Kerr–Newman metrics Reissner–Nordström metric inverse or conjugate or dual metric Induced...
  35. seabass101

    I Hubble deep field & ancient galaxies

    Hubble deep field allowed us to study galaxy evolution from 500 million years onward. Based on my (limited) understanding, I would expect ancient galaxies to contain fewer heavy elements and to have a more "juvenile" appearance, as compared to modern galaxies. Have we actually observed these...
  36. S

    I Models and theories of laws of physics emerging from chaos?

    Some physicists (like John A Wheeler, Holger B Nielsen or Ilya Prigogine) have proposed that all the laws of physics (including the most fundamental ones) emerged from a primordial chaos (for example, in the case of Wheeler, he proposed that laws of physics emerged from an initial random and...
  37. B

    B Did the Big Bang expand faster than light?

    I read that our telescopes pointing in any direction show light coming from the early days of the Big Bang (like 13.7 billion years ago). So did the expansion of the Big Bang to "fill" the universe happen faster than the speed of light?
  38. M

    I Shouldn't the Big bang be physically impossible?

    I would appreciate if someone could clarify this simple contradiction with the Big Bang. Assume black holes exist, and are caused by the collapse of large quantities of mass due to gravity. The resultant singularity is so massive that not even light can escape, much less material. Yet if the...
  39. S

    I Laws of physics from initial conditions?

    Are there any models, theories or physicists who propose that the fundamental laws of nature come from the initial conditions? Are there any physicists who propose that the most fundamental laws of physics emerged from initial conditions at the origin of the universe? And according to this view...
  40. N

    B Did the Big Bang follow multiple bangs? How did matter come to collide?

    I understand that the Big Bang was an explosion from an extremely tiny mass with particles and quanta traveling away into empty space (anything out there at the instant of this bang being too far to hit yet). I also understand that some ejecta from the Big Bang have been colliding since then...
  41. ChrisVer

    A An article on Big Bang theory contradictions

    Hi all, I was reading this article https://asiatimes.com/2020/11/the-big-bang-never-happened-but-fusion-will/ And I got somewhat confused. As most I've been taught that BB has succeeded in giving most of the cosmological predictions that we observe [nucleosynthesis, formation of galaxies, cmb...
  42. LCSphysicist

    I Book to study the big bang theory

    Could you recommend a book to study the big bang theory and the background necessary, mainly in math, to understand the theory rigorously?
  43. S

    I Schwartzchild radius and the Big Bang

    From a simple point of view, at the time of the big bang, the full energy/matter of the Universe should surely be within its Schwartzchild radius. Thus the entire Universe is within a huge black hole, and can never escape (i.e. is closed). Is this correct? What about the period of inflation...
  44. S

    I Bootstrap model and different laws?

    Physicist Geoffrey Chew proposed the concept of bootstrap (related to S-matrix theory) where he denied that fundamental laws of nature existed at all, as it is indicated in a writing in his memory written by one of his collaborators ([https://www.fritjofcapra.net/in-memoriam-geoffrey-chew/])...
  45. I

    I Laws Before Big Bang or After Big Bang

    Interested in thoughts... Did the laws of the universe exist before inflation phase of the 'big bang' allowing the 'big bang' to evolve, or did the laws evolve later...
  46. thegroundhog

    I Big Bang: Size & Matter of Universe Explained

    I've read that before the big bang all the matter in the universe was contained within an impossibly small space. How can you have matter in a smaller space than if all the space was squeezed out of an atom (or probability cloud if you want to be pedantic). Also, how does it fit it with Pauli's...
  47. K

    B Is the Big Bang only about the observable Universe?

    I was watching a video where Lawrence Krauss describes the big bang in terms of the observable universe. He says regions outside the observable universe need not have come from the big bang. Starts At minute 3.
  48. S

    I Wheeler's Mutability principle and multiple universes?

    Physicist John A Wheeler proposed the "Principle of mutability" which said that it could be the case that the universe would eventually shrink in a "Big Crunch" and the be re-born in another Big Bang. He proposed that the laws of physics (even the considered most fundamental ones) would change...
  49. bland

    I Does 'many worlds', imply worlds before the Big Bang?

    I'm at this point because a whole bunch of audio books I've been indulging in all seem to be converging on Many Worlds, and this has been given extra authority now because Sean Carroll appears to be a convert. I used to wonder if this was actual physics or metaphysics, and I've given up asking...
  50. S

    A Hartle-Hawking sum over all possible metrics?

    Physicists Stephen W Hawking and James B Hartle 1 proposed that the universe, in its origins, had no boundary conditions both in space and time. To do that, they proposed a sum over all compact euclidean compact metrics. I have heard that they only considered these metrics in order to simplify...