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.
We have set the Earths' Voltage at zero volts but is this 'earth voltage' applicable throughout the Universe. If not, is it relative in the same manner as spacetime and is it calculatable?
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...
Is it possible that our 3D universe is just a shadow in 3D space of 4D or higher dimensional reality? And we are limited in our perception of it by our 3-dimensionality. In other words like Flatlanders would just see s square of a cube sitting on the plane or passing through the plane...
We have a Universe that can be seen by Hubble maximum and we can imagine millions time more than that but if there is no boundary, there may be another millions of universe. Suppose we gather this all millions of universe and say this is a giant universe of universes and thus matter do not stop...
This idea has always bugged me:
If we are looking at the past when observing the redshift of far away galaxies (ex: 10 billions light years).. then how can we tell that the universe at the present time is still expanding at same rate, deaccelerated, or stopped expanding?
Does the relative density of the early universe contribute to the red-shift of distant galaxies?
If so, by how much? How would this be calculated?
Asked another way :
Assuming both the early universe and the current universe are flat, could the relative difference of their space time metric...
This is a question I was looking at based on Relativity and John Wheeler's one-electron universe theory.
https://en.wikipedia.org/wiki/One-electron_universe
My question is this. The faster you move towards the speed of light, wouldn't everything in the universe contract to a single particle...
I assume that before the Big Bang, there was no Higgs Field, since there was no universe for it to fill.
I assume that at the moment of the Big Bang, it began to seep into every corner of the expanding universe and was carried by inflation, that is, moving faster than the speed of light by the...
If you guys haven’t noticed through my posts so far is that I am interested in the Big Bang.
I am considering the concept of the Planck length, but I may have some confusion.
How did Space time start? Did the universe start from nothing and created units of spacetime? Could the original size...
It’s confusing how a universe can have finite mass but infinite size. Could the universe be like a tesselation, where you always end up where you started? Not like a closed universe, but where space is just copied over and over?
Hello,
this idea of holographic universe is mind boggling to me.
If we are 2d and everything we see is actully 2d hologram, like picture on the monitor
https://metro.co.uk/2017/01/30/our-entire-universe-is-an-illusion-and-reality-is-actually-a-2d-hologram-say-scientists-6415724/.
There is...
I have read that the early universe had a very low entropy. I don't understand why. A giant ball of plasma at billions of degrees K with particles moving in all directions. It seems like the definition of total disorder. Why is the entropy considered low?
Ok, so just a quick question, first law of thermodynamics basically states that matter cannot be created nor destroyed aka it can only change states and turn into energy or vice versa, the second law states that, IIRC, for perfect isolated systems entropy remains constant but for our universe...
Hi,
I was watching a documentary by Professor Brian Cox and he said human life is only possible for 10-86% of the life of the universe. He based this on the fact that the age of starlight we are currently in is a very small event in the life span of the universe and human life obviously can’t...
I was just curious because if space can expand faster than light, doesn't that mean there will be a lot of space that we just can't see? Do objects just vanish because we can't see them?
For instance, if a hypothetical alien lived in MACS0647-JD galaxy which is 13.3 billion light years away...
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...
IMO, it is very anthropocentric to specify that the Universe is 13.7 Billions years old, i.e., using Earth's period of revolution around a minor sun as a baseline. There might exist a Galactic timeframe that would/could suggest that we actually live in a very "young" Universe. Just for fun, I...
Hi Forum,
I've been pondering about this for a while and I hope someone here can help!
Imagine two bodies orbiting each other in tidal locking (https://en.wikipedia.org/wiki/Tidal_locking) just like Pluto and Charon, or as Earth and a geostationary satellite.
Nothing else exists in this...
Homework Statement
Consider the Godel Metric in spherical coordinates as on page 6 here;
ds^2=4a^2\left[-dt^2+dr^2+dz^2-(\sinh^{4}(r)-\sinh^{2}(r))d\phi^2+2\sqrt{2}\sinh^{2}(r)dt d\phi)\right]
This is a solution to Einstein's Equations if we have ##a=\frac{1}{2\sqrt{2\pi\rho}}## and ##\Lambda...
I read somewhere that it is not possible to prove that Our universe (from big bang to now) have different principles that inside of black hole. I think that one physicist said something like this.
Or in different words: are principles of our universe different as inside of a black hole?
Hello everyone,
I don't normally come to Astro/Cosmo forums, but I stumbled upon a discussion between a mentor and a PF member here, which involved explanations on the geometry of the universe: difference between boundless vs unlimited, 2-torus vs 3-torus, why are tori boundless, etc. This got...
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
Hello.Long time,No See.
I am just wondering if anyone can point be in the correct direction. Preferably with a link or a book that I should read as i would like to understand, not just know the answer.
I am just wondering how much the number of stars in the universe has changed over time...
A black dwarf is the dead ember of what once was an average star which expired as a white dwarf.
Is it possible that these may account for at least some of the dark matter?
What is the deviation in the expansion of the universe exactly quantified, when I would assume general relativity and project it backwards?
As a statistician I am asking for data, for either the backwards projected general relativity case and either the real expansion case, as it is...
I know this is getting into the highly speculative, but given the mathematics that has been done on versions of these concepts that are not known to be inconsistent:
In the inflation concept which leads to a multiverse of the "bubble universes" type, in which the laws governing the space-time...
I don't understand why CP violation is insufficient to explain the observed baryon asymmetry?
in every article I find it says: "we know its insufficient..." without an explanation.
I will be glad for an explanation and for articles that deal with this issue.
And how do you solve it using the...
Conceptually, at least, this is a simple question, although I recognize that it might be hard to calculate in practice from available data.
The matter-energy budget of the universe is measured (in a model dependent way) to consist of a certain percentage of dark energy, a certain percentage of...
My somewhat ropey understanding of entropy is that it is a measure of order/disorder and that in a closed system entropy always increases. Was discussing it with my teenage daughter. Whilst trying to convey my limited understanding it struck me that if the universe is contracting (we had also...
General Relativity equations tells us that the earliest time of the universe which our physics can tell us had infinite space and infinite density (i.e. matter).
Then space started expanding, thus increasing the distance of any 2 points of that infinite dense matter, thus making it less dense...
<< 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...
I've read that until 5 billion years ago the rate of expansion of space was decreasing. How do we know that it was slowing down?
Does the uniformity of the CMB mean that the rate of expansion is identical throughout the entire universe - or does it just mean that the average rate of expansion...
If the universe is very large relative to the observable universe and it is spherical, and the observable universe is well away from the outside region of the sphere, more towards the center, is spacetime approaching flat for the observable universe?
I always assumed the answer is yes, but then...
I just heard that the universe inflated from the size of an atom to the size of our solar system in 100 seconds. Wouldn't that exceed the speed of light?
Let me quote Stephen Hawking who sadly just passed away, in his book A Brief History of Time:
The total energy of the universe is exactly zero. The matter in the universe is made out of positive energy. However, the matter is all attracting itself by gravity.
Two pieces of matter that are...
The hyperboloid with equation: ## z^2=x^2+y^2-1 ##, embedded in standard 3-D Minkowski space ( +, +, - ) so that ## z^2 ## is negative, has positive Gaussian curvature equal to 1 ( as found directly from its metric: ## ds^2 = \sqrt{ -dτ^2+(Coshτ)^2 dθ^2 } ## induced from the ambient Minkowski...
Hey. I was just wondering what people mean when they say physics and the universe is computable, I always thought it meant that the universe is measurable/calculable, but someone said to me that it meant the universe works like a computer/simulation. So, what does it actually mean?
Is there empirical evidence demonstrating that the entire universe and the known (directly) observable universe are one and the same? Is there definitive evidence that what we observe (directly) is simply not a small patch of something much larger? That instead of our observations...
Good morning.
The question of the "continuous" or "discrete" nature of the universe is the subject of diatribe among the greatest physicists in the world. I would like to discuss the same topic, but asking a question about the aspect of continuum in classical mechanics.
The use of mathematical...
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...
If the universe keeps expanding and eventually ends in a "big freeze" or heat death, does this contradict the third law of thermodynamics?
The third law of thermodynamics states that a crystal at absolute zero has zero entropy. Since the entropy of the universe can never decrease, as the age...
Now, we all know all physical things are related to math. EVERYTHING is related to math. Now, we have all heard the theory that we are all living in a simulation, right? What if... math is the coding of the simulation and throughout time we have been decoding our own simulation.
The universe seems to be expanding since the farther away an object is, the faster it is moving. However, because of the finite speed of light, the farther away we look in distance, the further back in time we look. Does that mean that galaxies were moving faster in the past and are now slowing...
If the universe was in an energy eigenstate then d<A>/dt = 0 for any dynamic variable A. Stuff moves which implies that the Universe isn't in an eigenstate. What factors drive the energy spread?
If a big bang could occur at a point in space-time, couldn't we at least speculate that many big bangs could have occurred at many other points in space time, and could have resulted in many parrelel universes evolving far away or in close neighborhood of our own one?
And then, could it not be...
Penrose's CCC model posits a mapping between our big bang and a future one. This is based on the idea than in the far future there will be no mass and at the big bang there was no mass. So in both cases the universe looses track of scale. I am aware that the idea of there being no mass in the...