# Universe Definition and 127 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. ### B Gravity to infinity?

Hi, mathematically in the F = GMm/r^2 equation r can be very close to infinity (or the size of the universe), but gravitational force always will be some number. But how is that in the real world? Let's say we have a perfectly empty universe but only with two sun-like stars. If they are away...
2. ### 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...
3. ### I How many apparent horizons could the Universe have?

I was reading a paper written by George Smoot [1], which assumes the holographic principle as true and conjectures that our universe would be encoded on the "surface" of an apparent horizon as the weighted average of all possible histories. In that way, there would be one world (or universe)...
4. ### 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...
5. ### I Changing or breaking the most fundamental laws and symmetries?

There are some theoretical processes (like vacuum decay in quantum field theory) that could change the physical constants of the universe. Similarly, in inflation theory, various models predict that multiple regions that would stop inflating would become "bubble universes" perhaps with different...
6. ### I Gaia vs DESI star map

I've read that ESA's Gaia space observatory has created the "richest map of our galaxy (and beyond)" https://www.esa.int/Science_Exploration/Space_Science/Gaia/Gaia_creates_richest_star_map_of_our_Galaxy_and_beyond Meanwhile, it's been widely reported tat the "The Dark Energy Spectroscopic...
7. ### 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...
8. ### 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...
9. ### B What is the Universe expanding into?

As we know universe is everything having every matter and energy. From the observation using Hubble's law it was said that the galaxies are moving away from the Earth which means the universe is expanding. If it is everything then what is it expanding into. Is there some outer edge that it can...
10. ### 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...
11. ### I Evolving laws of physics?

The Canadian physicist, Lee Smolin proposed in the 90's that laws of nature may evolve and change over time. He begun proposing that only the constants of nature would evolve, while the most fundamental laws would remain the same, but recently he has change a little bit his ideas and has...
12. ### I What does Causality really mean?

In the wikipedia article of the observable universe I have read the following: "Both popular and professional research articles in cosmology often use the term "universe" to mean "observable universe". This can be justified on the grounds that we can never know anything by direct...
13. ### B The infinite and the infinitesimal

Over the years the following has continued to be my biggest question in Cosmology. In the past couple of years I wondered if we have got any closer to understanding whether our space is infinite or infinitesimal? (By infinitesimal I mean that there is no lower limit to the minimum separation of...
14. ### 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...
15. ### B How does the observable Universe have meaning?

Why does the term observerable universe have any meaning outside of observers on earth? From earth, the observable universe is the universe 13.8 billion light years away in every direction which is large but a finite distance. Say there's a galaxy near the edge of our observable universe...
16. ### I Physicists who propose that symmetries are emergent?

I know of some physicists (e.g Holger B Nielsen, Grigory Volovik or Edward Witten) who have proposed that all symmetries (Local gauge symmetries associated with forces and dynamics and global symmetries associated with conservation laws) are emergent rather than fundamental. Are there any other...
17. ### B Expansion of the Universe and the fourth dimension

The perimeter of a circle increases by radius, the surface area of a ball increase by radius(which is height which is the third dimension if the ball is a planet like the Earth), and the universe is expanding by time, can we say that the fourth dimension is time by this ?
18. ### I Why does the Universe have so many galaxies?

I know this question doesn’t really make sense but the absurd enormity of the universe always made my head hurt. A single galaxy alone has more then enough stars to satisfy the curiosity of any sentient being. Why 2 trillion? It’s just insane. Why did nature have to create so many? The spaces...
19. ### B How constrained are the initial conditions of the universe?

Do we know? Do we have any idea? There seems to be 2 schools of thought. 1. The initial conditions can be almost any value with universes inflating with different laws of physics. This is the multiverse and string theory 10^500 false vacua view. 2. The initial conditions are more restrained...
20. ### I Could fundamental laws change in Dirac's Large Numbers Hypothesis?

Paul Dirac proposed a hypothesis called "Large Numbers Hypothesis" (https://en.wikipedia.org/wiki/Dirac_large_numbers_hypothesis), where he basically stated that, if he was correct, laws of physics would change with time. But what about fundamental laws and constants? (Not only 'effective'...
21. ### I On Hubble's constant, the expansion of the Universe and vacuum energy

Recently I was thinking about the Hubble's constant (which, actually, is not Hubble's and not constant...) and wondering: if the universe is expanding at 70 km/s each Mpc, then there's possible to calculate some expansion of space, say from me to a person 1 meter away from me (theoretically)...
22. ### I Universe Expansion

So the universe is expanding, and galaxies are getting farther apart from one another on average. Does this motion count the same as ordinary motion, in that if a galaxy is being expanded away from us at 0.5c, that clocks in that galaxy would appear to tick slower at 0.866 the rate of clocks here?
23. ### I The laws of thermodynamics and the Universe

How would the first and second laws of thermodynamics apply to the creation and existence of the universe? I'm not a physicist (and unfortunately, do not remember a lot that I learned in Physics class in school and college about Thermodynamics). I did some searching and I have come across an...
24. ### The second law of the thermodynamics and philosophy

The second law of thermodynamics tells us that the amount of useful energy in an isolated system tends to decrease. Does this imply that mankind will reach a point where it cannot longer use any kind of energy? If so, do we have an estimate of how long it will be necessary for to haven't useful...
25. ### Friedmann's equation for a^3 with Λ, ρm

We need to prove that a3(t)= ρo/2Λ [cosh(sqrt(24πGΛ)*t) -1] by changing into a variable of u, where u=2Λa3/ρo From Friedmann's second equation we know that Λ= ρm/ 2 Also ρm= ρo/ a3 [First attempt] I begin from Friedmann's equation where (for here), ρtotal= ρm + Λ and k=0; a'2/a2 = 8πG(ρm +...
26. ### A Volume of a closed Universe

The final result must be V=2π2α3 Hint says we must use the dV in the spherical system (dV=r2sin2θdrdθdφ) as well as the equation of the three-dimensional metric ds2= c2dt2 - a2[ dr2/(1-kr2) +r2(dθ2 +sin2θ dφ2) ] For a closed universe we know k=+1 and with dt=0 My problem is, I don't understand...
27. ### 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. ### I Collision of Milky Way and Andromeda....?

I do not understand how (in few bilion years) Milky Way and Andromeda will collide? If the universe is expanding and these objects which are far away have bigger speeds than those who are closer, than Milky Way and Andromeda will just be far away every moment... (?) Could someone help? :-)
29. ### Is the mass of the electron constant?

If the mass of the electron has been changing during the evolution of the universe, then the orbits of the electrons would also change, which will shift the light spectrum of each atom. Could this explain red shift of far galaxies, and the shift is not because the universe is expanding? Henry
30. ### B Why the multiverse makes no sense to me

So, basically, multiverse make sense because we can't imagine our universe standing in nothing(or being everything that just expands itself). If we take this principle, we should have a multi-multiverse, a multi-multi-multiverse and so on. (When the chain ends?). Is it more evidence to support...
31. ### B What would the universe be like with less mass and energy?

So I have sort of a conceptual question about the big bang and gravity. Imagine yourself in a universe, in which existed about the number of particles/energy in a 3X3 metre room at any given moment. This universe has the same laws of physics, constants and is identical in every way to our...
32. ### I What is the acceleration of the universe?

When I think about it, I see galaxies getting farther away with a constant acceleration. An acceleration that must be caused by a force ( pinned Dark Energy ). I lack in knowledge of the subject but yet I wonder what is the value of the acceleration in the expansion of the universe. Just like in...
33. ### B Was the Big Bang simply "Distant Traffic"?

We keep hearing that "It all started with the Big Bang" and how "Everything was compressed into an infinitely small dot" and suddenly it expanded. Personally, I've wondered if this is more of a misinterpretation of distance, just as the headlights of heavy traffic on the highway looks like a...
34. ### I History of the Universe and measured outcomes

This thread is a split-off of this post: https://www.physicsforums.com/threads/do-macro-objects-get-entangled.946927/page-2#post-5997089 So my issue is this: if, for convenience, we use a Copenhagen interpretation, and we measure an observable WF ##\alpha |A \rangle + \beta |B \rangle##, then...
35. ### A This is the most active place in the universe

According to Source: They discover a mega cluster of 14 galaxies that originated only 1400 years after the Big Bang This is the most active place in this universe
36. ### Insights Intro to Big Bang and Infinity Concepts - Comments

Greg Bernhardt submitted a new PF Insights post Intro to Big Bang and Infinity Concepts Continue reading the Original PF Insights Post.
37. ### A How can the universe keep on expanding if it's infinite?

How can the universe keep on expanding if it's infinite? Expanding metal, like a cube of aluminium, has a surface area which forms a border for the matter contained inside. So the universe must have a border for the matter it contains.
38. ### I Making Wormholes

Is there a possibility that we create a wormhole sometime and switch between universes? First of all what is required to make a wormhole? What will happen if we made a wormhole? What proves that there are many universes and that we can switch between them??
39. ### Insights A Journey Into the Cosmos - FLRW Metric and The Friedmann Equation - Comments

Greg Bernhardt submitted a new PF Insights post This article is part of our student writer series. The writer Arman777, is an undergraduate physics student at METU A Journey Into the Cosmos - FLRW Metric and The Friedmann Equation Continue reading the Original PF Insights Post.
40. ### 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.
41. ### Insights A Journey Into the Cosmos - The Friedmann Equation - Comments

Greg Bernhardt submitted a new PF Insights post A Journey Into the Cosmos - The Friedmann Equation Continue reading the Original PF Insights Post.
42. ### B Correlation between black holes and parallel universes.

I'd like to know people's views on the possibility of Black Holes being 'Gateways' or 'Portals' to parallel universes.
43. ### I Entropy of the last scattering surface and today's universe?

Hi, I am quite confused about followed question, I think scientist think the last scattering surface was dense plasma at the temperature of 3000K. If the today's universe much cooler and less dense then "the last scattering surface" how can anyone says entropy increased by time? Isn't universe...
44. O

### B Age of the universe: observable or entire universe?

I was wondering if we are only aware of the observable universe, and the actual universe could be infinite, what do we mean when we say the age of the universe is 14 billion years? Are we talking about the age of only the observable universe or the whole universe?
45. P

### I IOAA 2nd problems: universe expansion

Could anyone please guide me how to do these problems?
46. ### I A copy of me in an infinite universe

This question on quora discusses whether an infinite universe would allow for a repetition of patterns and arrangements of matter. The top answer gives a very convincing argument as to why this would be impossible because, he reasons, that spacetime is causally connected. However if the universe...
47. ### Insights A Poor Man's CMB Primer. Part 5: Quantum Seeds - Comments

bapowell submitted a new PF Insights post A Poor Man's CMB Primer. Part 5: Quantum Seeds Continue reading the Original PF Insights Post.
48. ### I Homogeneous static universe

Consider an infinite homogeneous static universe with a constant mass density $$\rho$$. If we were to calculate the force on a test particle located at a certain point accoring to Newtons law of gravity. It would be logical to conclude from a symmetry argument that the force on the particle...
49. ### I Math doesn't add up: stars + planets < particles in universe?

I see they finally counted all the particles in the universe, it's10 to power of 80 They also counted all the planets: 10 to power of 24 and also counted all the stars: 10 to power of 24 and also counted all the atoms in the Earth: 1.3 x 10 power of 50 Good work everyone! Now, I'm trying my...
50. ### How can thermal fluctuations decrease entropy?

https://en.wikipedia.org/wiki/Future_of_an_expanding_universe "Over an infinite time there could be a spontaneous entropy decrease, by a Poincaré recurrence or through thermal fluctuations (see also fluctuation theorem)"