What is Universe: Definition and 996 Discussions

The universe (Latin: universus) is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. According to estimation of this theory, space and time emerged together 13.799±0.021 billion years ago, and the universe has been expanding ever since. While the spatial size of the entire universe is unknown, the cosmic inflation equation indicates that it must have a minimum diameter of 23 trillion light years, and it is possible to measure the size of the observable universe, which is approximately 93 billion light-years in diameter at the present day.
The earliest cosmological models of the universe were developed by ancient Greek and Indian philosophers and were geocentric, placing Earth at the center. Over the centuries, more precise astronomical observations led Nicolaus Copernicus to develop the heliocentric model with the Sun at the center of the Solar System. In developing the law of universal gravitation, Isaac Newton built upon Copernicus's work as well as Johannes Kepler's laws of planetary motion and observations by Tycho Brahe.
Further observational improvements led to the realization that the Sun is one of hundreds of billions of stars in the Milky Way, which is one of a few hundred billion galaxies in the universe. Many of the stars in galaxy have planets. At the largest scale, galaxies are distributed uniformly and the same in all directions, meaning that the universe has neither an edge nor a center. At smaller scales, galaxies are distributed in clusters and superclusters which form immense filaments and voids in space, creating a vast foam-like structure. Discoveries in the early 20th century have suggested that the universe had a beginning and that space has been expanding since then at an increasing rate.According to the Big Bang theory, the energy and matter initially present have become less dense as the universe expanded. After an initial accelerated expansion called the inflationary epoch at around 10−32 seconds, and the separation of the four known fundamental forces, the universe gradually cooled and continued to expand, allowing the first subatomic particles and simple atoms to form. Dark matter gradually gathered, forming a foam-like structure of filaments and voids under the influence of gravity. Giant clouds of hydrogen and helium were gradually drawn to the places where dark matter was most dense, forming the first galaxies, stars, and everything else seen today.
From studying the movement of galaxies, it has been discovered that the universe contains much more matter than is accounted for by visible objects; stars, galaxies, nebulas and interstellar gas. This unseen matter is known as dark matter (dark means that there is a wide range of strong indirect evidence that it exists, but we have not yet detected it directly). The ΛCDM model is the most widely accepted model of the universe. It suggests that about 69.2%±1.2% [2015] of the mass and energy in the universe is a cosmological constant (or, in extensions to ΛCDM, other forms of dark energy, such as a scalar field) which is responsible for the current expansion of space, and about 25.8%±1.1% [2015] is dark matter. Ordinary ('baryonic') matter is therefore only 4.84%±0.1% [2015] of the physical universe. Stars, planets, and visible gas clouds only form about 6% of the ordinary matter.There are many competing hypotheses about the ultimate fate of the universe and about what, if anything, preceded the Big Bang, while other physicists and philosophers refuse to speculate, doubting that information about prior states will ever be accessible. Some physicists have suggested various multiverse hypotheses, in which our universe might be one among many universes that likewise exist.

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  1. U

    B Do we 100% know what the geometry of the Universe is?

    If big bang existed, than universe must be sphere, because explosion expand in all direction.. I read that universe is maybe flat so how this is possible? Why can we with telescope determine shape of universe?
  2. mister i

    B Can the "creation" of space in the Universe affect the speed of light?

    Can the expansion/creation of space in the universe affect the speed of light over long distances? (dividing the space traveled (including expanded space) by the travel time). If the Hubble constant is valid throughout the universe, it follows that from a certain Megaparsec of distance, galaxies...
  3. mister i

    B Does the age of the universe differ for observers in expanding space?

    The age of the universe is said to be about 13.8 billion years. But, since time depends on the observer, would it be the same for a possible inhabitant of a planet in a galaxy about 10,000 million light years away that is separating from us at 60% of the speed of light?
  4. E

    I universe is older than we think

    We think the age of universe is 13.8 billion years old . I think we are wrong. Let’s pretend we can go outside of our observable universe. Also, let’s say we can go so far out that we do not even see a spec of light from our universe. We see nothing at all, total darkness. When they finally do...
  5. P

    I Why didn't the Big Bang form a Black Hole?

    (pop science) It seems like the mass in the universe at the point of origin was way higher and the size way smaller than required to form a black hole, so why didn't our universe just sit forever as a single black hole? From a figure for the mass of the universe you could calculate the event...
  6. Z

    B Number of atoms in the Universe

    hi, how do we calculated the number of atoms at the start of universe i.e, after big bang?
  7. L

    B Does the Universe prefer certain kinds of matter based on the laws in place?

    My understanding is when a star creates new matter it has an affinity towards iron? Does the universe on a whole favor certain elements based on the laws in place? Certain complexity level and certain physical characteristics are ideal and the farthest from those things the stronger the force to...
  8. 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
  9. KDP

    I Brian Cox and the end of the Universe?

    Professor Brian Cox was on the TV last night. He stated that eventually everything will end up in black holes. When there is nothing left to absorb they will start evaporating via Hawking radiation until eventually they all disappear in a small flash of light and then there will be eternal...
  10. S

    I All possible models to explain the hierarchy problem?

    There is an interesting paper by Arkani-Hamed and collaborators (https://arxiv.org/abs/1607.06821) to address the hierarchy problem. There, they consider many possible models of fundamental particle physics where they all have an exact copy of the Standard Model but with different Higgs...
  11. WesKnight

    B Questions about relative Space-Time in the Universe

    Hello everybody. I have been having a lingering question about the State of the Universe. As humans, we view the Universe's expanse as historical from the vantage point of Earth. For example, 10,000 light years away would have occurred 10,000 years ago. On that same logic, looking at Earth from...
  12. A

    I Why Is a Flat Universe Infinite?

    I cannot find an answer online. Help please.
  13. Fractal matter

    Is fractal universe with determinism possible?

    I'd like to know your opinion. Do you think the universe could posses the following qualities altogether? 1) All quantum fields are effective fields 2) Gravity is an effective field 3) Local realism (possibly superdeterminism) 4) There is a preferred frame of reference It seems to me, that...
  14. PhanthomJay

    I Unobservable Universe and the CMBR

    How can there be galaxies in the unobservable universe when The CMBR which precedes all galaxies is observable?
  15. T

    I Is anything not spinning in the Universe?

    so I've started doing a few equations involving time dilation due to gravity and have been told it changes depending on if its spinning or not (like with a black hole), how different is it, and whether there is any time something won't be spinning and it needing the altered calculations
  16. J

    B Expanding Universe -- What is it expanding into?

    I believe in the Big Bang but I struggle with something. If the Big Bang happened and universe is expanding there has to be something (or really nothing) there for the universe to be expanding into. Any thoughts?
  17. 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...
  18. S

    I Thoughts on this article: The Universe Is More in Our Hands Than Ever Before

  19. S

    I What is theorized to be the coldest white dwarf in the Universe?

    Yes, I understand that once white dwarfs get cold enough, they aren't luminous enough to get observed, so my question could be interpreted as "when did the earliest white dwarf evolve, and how cold is it now"? This is the article that motivated this question...
  20. J

    B How can the Universe be flat? Ridiculous

    The earth is a sphere. I can stand anywhere on its surface and look up directly above my head at the night sky and see lots of stars many light years away. I see this view wherever I stand on the earth's surface although the stars will be different ones, depending on where I'm standing...
  21. S

    I Topological phase transitions for the whole Universe...?

    Physicist Grigory Volovik has put forward some ideas about the universe undergoing a topological phase transition (especially in the early stages of the universe). He published a book called "*The Universe in a Helium Droplet*" where he explained his ideas. You can find a brief discussion here...
  22. 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...
  23. J

    I Universe Expansion: GR vs Hubble Reconciled

    The GR predictions for the universe's size are those of fig.a. Whereas the Hubble expansion is exponential, fig.b. How are the two reconciled?
  24. M

    I Is there any way in the future to determine the Universe's size?

    I am frequently contemplating the size of the universe. Logic tells me that existence cannot is end as there really can’t be anything as anti-existence but if the universe loops back on itself; it may not be an issue. Is there a possible way in the far future to semi-accurately measure its size...
  25. BadgerBadger92

    B Where did the first universe come from?

    If universes can collide to make universes and split to make new ones, where did the first universe come from? Do we have any ideas? Any help would be appreciated.
  26. F

    B Map of M87 Galaxy & Universe: Find & Explore Here!

    Does anyone know where I can find the map animation of the galaxy / nearby galaxy cluster that illustrate the geometry between Earth and M87 at the 29th minute in the Netflix show ? Alternatively if that's not a public source, can anyone recommend a publicly available "map of the universe", I'd...
  27. S

    I Inhomogeneities and topological defects in cosmology...

    I have heard that some types of inhomogeneties and topological defects (like cosmic strings) in cosmology have been proposed to be able to break fundamental symmetries of nature such as the Poincaré, Lorentz, diffeomorphism CPT, spatial/time translational...etc symmetries... However, I have not...
  28. S

    I What does it mean that symmetries do not hold globally?

    Perhaps this is a stupid question but, if Lorentz symmetry and time translational symmetry are not global in an expanding universe, wouldn't that mean that is possible that other Hubble spheres outside our observable universe could have other symmetries or an absence of the Lorentz symmetry? I...
  29. Jaime Rudas

    I The curvature and size of the Universe

    According to the Planck 2018 results, the curvature component of the density parameter of the universe is Ωκ=0.001±0.002. From this data, would it be possible to determine the greatest possible positive curvature of the universe and the radius of the corresponding 3-sphere?
  30. gleem

    I Massive galaxies during the early Universe, new JWST data

    As of now, it appears the ΛCDM can accommodate this new data but new data is needed to be sure. https://www.quantamagazine.org/standard-model-of-cosmology-survives-jwsts-surprising-finds-20230120/
  31. M

    I Expansion of the Universe and the cosmological principle

    Wikipedia states the following in their article about the expansion of the universe: If the cosmological principle was discovered to be false in our universe, i.e. our universe was discovered to be inhomogeneous or anisotropic or both on very large scales and the FLRW metric does not hold for...
  32. A

    Exploration of an Artificial Universe: Properties and Behaviors

    TL;DR Summary: I'm interested in constructing an artificial universe and discussing its properties and behaviors. I would like to construct an artificial universe somewhat akin the Conway's Game of Life and discuss its behavior and properties. Is that allowed? Is this the place to do that...
  33. Ivan Seeking

    Schwarzschild radius of the Universe

    https://www.msn.com/en-us/news/technology/the-astonishing-scientific-theory-that-says-the-universe-might-be-inside-a-black-hole/ar-AA17lxtF?ocid=msedgdhp&pc=U531&cvid=272cb184de9c48fbbd3b321120e37dac Michio Kaku has often joked, "If you want to know what it looks like inside of a black hole...
  34. Feynstein100

    I Black Holes: Event Horizon Causally Disconnected?

    I've heard quite frequently that events inside the event horizon of a black hole are causally disconnected from the rest of the universe. I take it to mean that while outside events can interact with the events inside of the horizon, the reverse is not true i.e. inside events cannot interact...
  35. Pushoam

    I Conservation of charge in the Universe

    The charge of an isolated system is conserved. This implies the charge of the universe is constant. This implies that charge can neither be created nor destroyed. This implies that the net positive charge and the net negative charge of the universe are conserved. Is this right?
  36. V

    A Can an Oscillating Eternal Universe be Described without Singularity?

    Consider the FLRW metric. We pick a specific definition for the scale factor as suggested bellow. Suppose we have a hypothetical metric having the scale factor defined by ## a(t)=\sin(t) (1+ \text {sgn}(\sin(t)) +\epsilon ## Does this make sense, mathematically (and physically)? Like having...
  37. W

    B Question regarding supermassive black holes in the early Universe

    I'm really curious about this, but I want to know how wrong I am. I've seen in a lot of content recently about how observations of early supermassive blackholes are observed to be more massive than they should be. If I understand it correctly it has to do with the maximum rate a super massive...
  38. V

    I Uncertainty Principle in QFT & Early Universe Conditions

    I have a question related to the uncertainty principle in QFT and if it is related to the early universe conditions. Do we still have four-vector momentum and position uncertainty relation in relativistic quantum theory? I have been following the argument related to the early universe and the...
  39. H

    B Why so many want, or are not disturbed by, a "deterministic" universe?

    Determinism, as I understand it, means that everything that can ever happen could, has already been determined, because everything is just a reaction based on everything else. With good enough instruments you could predict with 100% accuracy the future of everything in the universe. Quantum...
  40. Chunkythunk

    B Is the Universe really infinite?

    I believe the universe could not possibly be infinite. I am not denying it is of extremely large scale but it still must be finite. No true infinity is found in nature but only in mathematics. why should it differ here ?
  41. 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...
  42. Slavik Komarova

    I Any evidence of white holes in the Universe?

    We have much evidence of the existence of black holes in our universe ... so why does not the same occur in relation to white holes since they are also the result of the same theoretical prediction ?
  43. KobiashiBooBoo

    B Age of Universe: Big Bang, Braneworlds & Perspective

    Although I understand that time does not exist prior to the big bang, it's still a difficult concept that the universe - the sum of all that is and ever will be - has a beginnnig and therefore an "age." While we could say that time as a dimension began at the big bang, and it was meaningless...
  44. S

    I Understanding the Zero Order Universe in Cosmology

    In chapter 2 of Dodelson’s modern cosmology book, it reads: “Implicit in this discussion will be the notion that the universe is smooth (none of the densities vary in space) and in equilibrium (the consequences of which will be explored in Section 2.3). In succeeding chapters, we will see that...
  45. alan123hk

    B Is A Universe w/o Mass Possible? Exploring Time & Matter

    Is it possible to have a universe with only space and time but no mass ? I ask this question because a friend told me that time is an illusion. In fact, time does not exist. Because of the existence of matter, time can be felt through the movement of matter. If matter does not exist, time does...
  46. F

    B Does a contracting universe destroy its black holes?

    I found related questions being debated on the web so i'm not sure wether the question is closed. The following simple reasoning seems to imply that indeed the contracting universe is able to destroy its blackholes but what's wrong with it ?: The black hole solution is usually computed outside...
  47. T

    I Hubble Parameter as function of time in universe models

    This graph shows ##H## as a function of time related to the L-CDM model. Do we (@Jorrie) have similar graphs e.g. for ##\Lambda=0##; ##k=-1## critical, ##\Lambda=0##; ##k=0## open, ##\Lambda=0##; ##k=+1## closed? That would be great, thanks in advance.
  48. 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...
  49. Jimyoung

    Why is the Entropy of the Universe (total entropy) a path function?

    I understand that S (Ssys) is a state function but I can't understand why Ssurr and Suniv (or Stot) are a path function.
  50. 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...