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.
What are we going to do about it?
arXiv:1508.00631 [pdf, other]
The fate of a Universe driven by a linear potential
Ricardo Z. Ferreira, Pedro P. Avelino
Comments: 6 pages, 3 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology...
Can we really say that the universe is accelerating its expansion because we find that galaxies further away have more redshift? We know that we are looking back in time when we look at the stars, and when we look at the galaxies furthest away from us, we are seeing them as they were a long time...
Forgive my ignorance?.. If we can see 13.8-ish billion light years away how can the universe be the same age? Matter cannot travel at the speed of light, so how are we as far away (in light years) as the universe is old?
This is probably a silly question, but it's summertime and I can't ask my professor. If the universe turned out to be closed (even in light of WMAP's findings), would it be possible to consider the entire universe a closed system?
Dear PF Forum
Can I ask something quoted from a closed thread?
In a closed thread: https://www.physicsforums.com/threads/predicted-heat-death-of-the-universe.824652/
This quote was presented by a Brian Cox. A scientist? And if he is, I'd like to ask this question.
Is it technically possible...
Is ther a term for the space that the expanding universe has not reached yet. Is it called a void or is it some other term. Also if you could link some articles about this space the universe hasnt reached yet I would love to read about it. Thank you all.
In this documentary they discussed some research experiments which concluded that the universe is infinite. I didn't really understand it. Can someone explain how we know that the universe is infinite?
Wouldn't this also mean that the universe was infinite at the big bang?
I watched a BBC documentary that said that the observable universe is about 46 billion light years in size. How can this be if the age of the universe is 13.7 billion years (and nothing travels faster than the speed of light)?
So as time goes on there are more and more interactions in the world. That means that there are more wavefunction collapses going on. Some think that there is a universal wavefunction that is guiding all the individual wavefunctions. So I have to wonder if all these smaller, individual...
I accept that what I write below will be unpopular and argued with vehemently, however I think it should be considered with an open mind and recognised for the sense that it has.
Originally the concept of an expanding universe (and therefore by extrapolation the concept of the Big Bang that...
From what I understand, our Hubble's sphere is just relative to Earth and has a diameter of 93 billion light years putting the edge of the observable universe at 46-47 billion light years away.
So every object in space will essentially have it's own Hubble's sphere and objects near the edge of...
Predicted Heat Death of the Universe
In his thought provoking documentary “Wonder of the universe” (
) Professor Brian Cox explains the thinking behind current predictions for the “Heat death of the universe”. He explains the predictions through the effect of the second law of thermodynamics...
Hi all,
I was recently watching one of Susskind's 'Theoretical Minimum' lectures in which he says that the entropy of the universe may be measured via the number of observable photons, and that somehow these quantities (photon number and total entropy) are somehow linked. Could anybody with...
In wikipedia says Physical baryon density: ##Ω_bh^2=0.02230±0.00014## and
Physical dark matter density:##Ω_ch^2=0.1188±0.0010##
Matter density:##Ω_m=0.3089±0.0062##
so If we collect baryonic matter density and dark matter density we...
Is the U curved or are there new physics in play here. arXiv:1507.04684 (cross-list from gr-qc) [pdf, other]
Consistency of non-flat $Λ$CDM model with the new result from BOSS
Suresh Kumar
Comments: 5 pages, 1 figure
Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and...
Is every combination possible based off of uncertainty? When the big bang happens in another universe and one electron moves slightly differently because of uncertainty it changes everything. Is that why there are "infinite" number of universes?
When people say infinite do they really mean...
If you were to extend Born's Rule to sum or integrate over all points in space-time, would you necessarily get only either zero or one? Otherwise put, is it true that "anything that can happen will happen"?
Hi all. I am curious as to why quark stars have not dominated by now, given their inability to decay as they are a mass of fundamental particles. What prevents this?
I am trying to understand some things but I seem to be confused a little...
I was watching a video and it said when you look with hubbles telescope you see the past universe but I seem to have trouble understanding this, does that mean we could see the future if we knew which direction to point...
Hello,
I think it was Kurt Gödel who asked this while taking about time. The age of the universe and its size, is whatever it is for us because we are moving at a specific speed relative to light? If that is true, then a consciousness living on a planet moving at triple the speed of Earth...
An interesting discussion at this conference here:
http://physics.princeton.edu/cmb50/videos/20150612_session6_2.mp4
David Spergel said the next project should be to look for something called f and L or is is FNL?
Can someone explain what this is and what its significance is?
What did you guys...
I read in a book that there is no center of the universe because for example, you put dots around a balloon, and when you put air into the ballon, the dots will separate from each other, and each dot will see themselves as the center of the balloon, depending on the dots' perspective. That only...
This paper; http://arxiv.org/abs/1506.08573, First law of thermodynamics for dynamical apparent horizons and the entropy of Friedmann universes, offers a themodynamic evaluation of the apparent horizon of the observable universe. While not for the mathematically faint of heart, it is interesting...
Hello,
I've worked through most of Carroll's appendix on the non-coordinate basis.
I see and agree how the spin connection and tetrad one-forms are useful while calculating.
However as an example he sets out to apply the formalism to a spatially flat, expanding universe.
ds^2 = -dt^2...
Hey all,
I was reading a webpage which I found rather confusing:
http://www.dailygalaxy.com/my_weblog/2015/06/todays-galaxy-insight-the-lonely-galaxies.html
The problem I found with this is that the guy (who is a famous theoretical physicist) is stating that because the objects will be traveling...
I understand the concept of galaxies moving away from us at faster than the speed of light so that they lie outside of a "cosmic horizon" which we cannot see past. This would limit our observations to a "visible" universe which may be far smaller than what the "actual" universe may be. Alan...
I stumbled across this app for iPhone which may appeal to those favoring the many-worlds interpretation of QM:
http://aerfish.com/UniverseSplitter/index.html
I thought it was cool that an app can actually run a quantum experiment each time you use it. I saw mention of it on a Sean Carroll...
The Size and Age of Our Universe
If the universe is 14 billion years old, (roughly) then we can only see out in any direction for 14 billion light years. Thus any stars beyond 14 billion light years are invisible to us because their light hasn’t had time to reach us.
If we cannot find an edge...
I understand that the force of gravity prevents galaxies from expanding, as space increases. However, I question if universal expansion separates photons (electromagnetism), as they are traveling along parallel paths (Would the normal distance between them increase over time)? Thank you.
Is it true that the universe itself expands faster than c = 3 × 10^8 m/s at some places / areas / points.
I have heard it but wasn't able to confirm.
Because outside of the the universe, there ain't anything as far as we understand and hence the universe should not have to follow any law as...
Hello! My name is Brenna Bruce and I am 15 years old. This past year I have discovered my thirst for knowledge about the universe. How it works and what makes it work (to my understanding is explained by mostly physics) are concepts I want to be able to grasp. My question for anyone reading this...
Hello All :)
I've heard it proposed that a reason for the fine tuned nature of our universe in a way to allow the existence of complexity and life could be that the universe itself is the product of evolution in a multi-verse.
I don't want to debate around the anthropological principle as I am...
If we suppose universe is infinite than there will be no parallel universes.So I know that If ##Ω_k##→Universe will be infinite.Then there will be no parallel universes cause there's one universe.
I am confused.Is that mean the parallel universe idea is wrong ?
I was searching a cosmology calculator and In one calculator I saw this equation.
##1=Ω_k+Ω_m+Ω_Λ##
is that true true for all type universe's (open,flat,closed) ?
and is here ##Ω_k={-k\over H^2_0 /a^2_0}## isn't it.
(And If you can give me an article about this issue I will be...
Hello,
I just recently found out that one could find the Friedman's equation in Newton's approximation (without GR) by assuming that the universe in homogeneous and isotropic simply by using F=ma and the conservation of energy.
On can then find that the scale factor goes as t^2/3, as expected...
apologies if this has been asked before:
I'm trying to understand the expansion of the universe and i was wondering. . . . as the universe expands, are new points of space (or is it spacetime) being created?
if the answer is 'yes', what are the ramifications of this? my understanding...
I am looking for reliable information about the functional dependence of the diameter ##d(t)## of the visible universe on the time ##t## since the big bang singularity, based on the different hypotheses currently deemed competitive.
Is a new article, btw
June 2, 2015 Experiment Provides Further Evidence That Reality Doesn't Exist Until We Measure It
http://www.iflscience.com/physics/measurement-rules-quantum-universe
http://www.sciencealert.com/reality-doesn-t-exist-until-we-measure-it-quantum-experiment-confirms
any...
Anyone have any insight concerning the latest research underway at Heidelberg University... concerning the "smaller than once thought" earliest stars? Any thoughts on why these 'smaller than once thought stars' also contain carbon, when at this early stage of star formation in a very young...
I'm a retired geologist ... Know little about cosmology or quantum physics so will defer to others.
Please excuse my rambling question.
Something has always 'made me wonder' about the potential fate of the Universe. Almost all models predict either an infinite Universe or a Universe that...
I have a question :x
Technically, particles, like photons, which don't interact with the higgs field travel at the speed of light.
wouldn't the absolute minimum speed limit be something that interacts infinitely with the higgs field?
Dear PF Forum,
Sorry if I ask again, because I want a detailed information.
Does the universe has a frame of reference?
Two twins. A stays, B "travels". (Travel here is confusing, B will say I stay and A travels) and B "turns around" and come back.
B ages more slowly then A.
Q1: Why?
A1...
I'm 13, so I was reading Discover and I read something about a "block universe" and I thought it was a bunch of hogwash... here is how I see it... Time is the same for everybody its just how people perceive it that makes people think its an illusion... any feedback? Thanks!
Do systems further away from equilibrium increase entropy faster than a system with a high level of entropy and does this increase push the universe towards thermal equilibrium faster. Is there anything stopping the universe from reaching thermal equilibrium?
I've just watched the lecture of Penrose on his cyclic universe theory here:
I fact I understood that he claims that any kind of matter dissapears in a couple of Googol years due to Hawking radition; so there is no matter left at the end, which leads to a reduced degree of freedom in terms of...
Hubble observed that not only is the universe expanding but that it is accelerating. With the model of the Big Bang as I understand it, the universe began as a big explosion. That would mean that it expanded at a rate exceeding the speed of light or at least close to it. If that is correct, that...
Homework Statement
(a) Show the relation between frequency received and emitted
(b) Find the proper area of sphere
(c) Find ratio of fluxes
Homework EquationsThe Attempt at a Solution
Part (a)
Metric is ##ds^2 = -c^2dt^2 + a(t)^2 \left( \frac{dr^2}{1-kr^2}+ r^2(d\theta^2 + \sin^2\theta)...