What is Observable universe: Definition and 79 Discussions

The observable universe is a ball-shaped region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because the electromagnetic radiation from these objects has had time to reach the Solar System and Earth since the beginning of the cosmological expansion. There may be 2 trillion galaxies in the observable universe, although that number has recently been estimated at only several hundred billion based on new data from New Horizons. Assuming the universe is isotropic, the distance to the edge of the observable universe is roughly the same in every direction. That is, the observable universe has a spherical volume (a ball) centered on the observer. Every location in the universe has its own observable universe, which may or may not overlap with the one centered on Earth.
The word observable in this sense does not refer to the capability of modern technology to detect light or other information from an object, or whether there is anything to be detected. It refers to the physical limit created by the speed of light itself. No signal can travel faster than light, hence there is a maximum distance (called the particle horizon) beyond which nothing can be detected, as the signals could not have reached us yet. Sometimes astrophysicists distinguish between the visible universe, which includes only signals emitted since recombination (when hydrogen atoms were formed from protons and electrons and photons were emitted)—and the observable universe, which includes signals since the beginning of the cosmological expansion (the Big Bang in traditional physical cosmology, the end of the inflationary epoch in modern cosmology).
According to calculations, the current comoving distance—proper distance, which takes into account that the universe has expanded since the light was emitted—to particles from which the cosmic microwave background radiation (CMBR) was emitted, which represents the radius of the visible universe, is about 14.0 billion parsecs (about 45.7 billion light-years), while the comoving distance to the edge of the observable universe is about 14.3 billion parsecs (about 46.6 billion light-years), about 2% larger. The radius of the observable universe is therefore estimated to be about 46.5 billion light-years and its diameter about 28.5 gigaparsecs (93 billion light-years, or 8.8×1026 metres or 2.89×1027 feet), which equals 880 yottametres. The total mass of ordinary matter in the universe can be calculated using the critical density and the diameter of the observable universe to be about 1.5 × 1053 kg. In November 2018, astronomers reported that the extragalactic background light (EBL) amounted to 4 × 1084 photons.As the universe's expansion is accelerating, all currently observable objects, outside our local supercluster, will eventually appear to freeze in time, while emitting progressively redder and fainter light. For instance, objects with the current redshift z from 5 to 10 will remain observable for no more than 4–6 billion years. In addition, light emitted by objects currently situated beyond a certain comoving distance (currently about 19 billion parsecs) will never reach Earth.

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  1. KobiashiBooBoo

    B Observable universe and overlapping spheres

    We observe a very distant galaxy, thanks to the JWST. That galaxy has the same laws of physics as we do. Now imagine yourself in that galaxy, in your observable sphere of the universe, using your own telescope, looking in a direction opposite our current one from earth. Would we be able to...
  2. DavidCummings

    I Did James Webb cast serious doubt on the Big Bang?

    If you read articles like The James Webb Space Telescope prompts a rethink of how galaxies form you see that recent images of early galaxies have thrown some doubt on theories of how galaxies have formed and when they started forming. But if you read about the JWST in the popular media -- and...
  3. D

    B Would light travel in a circle at the edge of the observable Universe?

    Is there sufficient mass within the observable universe’s volume to form a black hole event horizon around the observable universe and, if yes would light fired tangentially at the edge of our observable universe ever loop back around in a circle or spiral inwards?
  4. Cerenkov

    B How can the Observable Universe be a closed system?

    https://www.physicsforums.com/threads/is-the-universe-a-closed-system.620503/ Hello. I was looking back at this thread from 2012 and, to be honest, I'm a bit confused. Quoting Drakkith... To our knowledge it is. At minimum you could count the observable universe as a closed system because...
  5. allisrelative

    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...
  6. 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.
  7. C

    I Is the total energy of the observable universe constant?

    If so, do we have estimates of what that is. Also, if we have estimates of that, what will the final entropy of the universe be, in J/K?
  8. H

    B What is the size of the observable Universe?

    Summary: What is the size of the observable universe and a bit of a rant. I've only recently jumped down the rabbit hole of physics. A social media post on time dilation four months ago got me hooked. Not only has it been a great way to exercise my brain, some of its discovery has been...
  9. Quantum Alchemy

    B A Question about space and the multiverse

    My question is about space and the multiverse. I was reading work by Max Tegmark and he sees the multiverse as Level 1-4. A Level 1 multiverse seems like it's self evident and I was wondering about the evidence against it. It's simple: Space expands faster than we can observe it so we exist in...
  10. platosuniverse

    B Doesn't there have to be more than one observable universe?

    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...
  11. T

    B The Universe vs Observable Universe

    After reading the wikipedia article and looking at many other threads on this forum I am still having a hard time understanding the difference between the Observable universe and the entire universe... Why is the entire universe not observable to us? The Big Bang happened 13.8 billion years...
  12. Arman777

    I Observable Universe Tempature and CMB

    It may sound stupid but something bothers me and I want to ask This question come to my mind due to another thread, https://www.physicsforums.com/threads/entropy-and-derivations-is-my-logic-faulty.935533/page-2#post-5910474 In Friedmann Equation we are assuming that universe is homogeneous and...
  13. durant35

    I Equilibrium of Universe: Earth vs. Black Hole

    I have a question regarding the process of getting towards equilibrium in our universe. If we imagine a causal patch with our planet at the centre, every planet will redshift away from us an after a while the planet itself will disintegrate, let's call this process the decay of Earth. Eventually...
  14. Chris Miller

    B Simultaneity in Non-Observable Universe: Is it Possible?

    Is there any method/model for determining simultaneity of events farther apart than light can ever travel? I'm guessing not. Would it be wrong to say that the non-observable universe exists only in our past (or future)?
  15. Arman777

    I Observable Universe Size in Different Perspectives

    I am reading The Essential Cosmic Perspective and there says "We cannot observe light coming from anything more then 14 billion-light-years away". ( In my opinion this statement is wrong cause observable universe diameter is 46.5 billion light years. I guess authors meant something else, or...
  16. I

    B Big Bang theory and the known universe

    I just want to verify from physicists whether what I have read in this article is true: http://www.astro.ucla.edu/~wright/infpoint.html It says: The Universe was not concentrated into a point at the time of the Big Bang. But the observable Universe was concentrated into a point
  17. R

    I Centre of mass of the observable universe

    Ok so I have read several of the threads regarding the impossibility of determining the centre of the universe based on observations of expansion of the universe. That one seems to have been beaten to death. So this is a slightly different question. When we look at all the matter that we can...
  18. Grinkle

    B What is meant by the size of the early observable universe?

    As a reference I will make the vague statement that others have posted the below on these forums: As one rolls back the clock, the size of the observable universe becomes smaller than it is today. Does this mean that all of what we observe today was more densely packed yesterday than it is...
  19. M

    I Object moving out of the Observable Universe due to Expansion

    With the LDCM, cosmological constant, model I understand that the scale factor of the Universe grows more rapidly than the Horizon. I believe the correct horizon I need to be considering is the Hubble Horizon and the point when objects recessional velocity hits the speed of light they disappear...
  20. R

    I If the observable universe was visible to the naked eye....

    What would the Earth's night sky look like? Would our eyes see any dark spots? Is there are way to calculate such a probability? This hypothetical is about the observable universe only. 93 billion light-years diameter, isotropic, visible light. Not sure what's the most appropriate tag for this...
  21. P

    I What is the size of the observable universe relative to CMBR

    Reading the Wikipedia article makes my head spin! Somehow the figure https://en.m.wikipedia.org/wiki/File:Observable_universe_logarithmic_illustration.png doesn't seem quite right. I don't understand how the CMBR doesn't define the edge of the universe. If we can see the CMBR that happened near...
  22. R

    I The observable Universe and its shape

    Laymen here. 1. From my understanding the universe is like the surface of a balloon. The universe is expanding as a balloon grows when air in being placed inside of it. Just like a surface of a balloon if you go in any direction in a straight line you will come back to the original point. Is...
  23. P

    Does age of Universe reveal its size?

    Does knowing the age of the universe (13.8 billion? ) say anything about size of the universe (besides the observable size)?
  24. Harley Cabral

    Help finding some info about the movement of the observable universe

    Hello all! Recently I watched a TV Series episode, but I didn't catch it from the beginning and I need some help to find which one was it, or any clue about it. The part that I want is about the Scientist guy explaining that things on the observable universe (big picture) seems to be moving...
  25. Buzz Bloom

    Derivation of Gott's formula for size of observable universe

    I understand that Gott derived a formula for calculating the size of the observable universe, and the value of the diameter based on current obsrvations is 93 Gly. Can someone please show the mathematical derivation of Gott's formula, or give a reference to a source which shows this derivation?
  26. G

    Age of the observable Universe?

    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?
  27. Rupert Young

    How can the observable universe be 46 billion lyrs in size?

    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)?
  28. quantumfunction

    How many Hubble Spheres are in the universe?

    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...
  29. R

    Newbie question.... The Big Bang & The Observable Universe (and time)

    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...
  30. K

    The Entire Observable Universe

    The entire observable universe is contained rather graciously in this wonderful picture, for your admiration.
  31. Stephanus

    Farthest Object in Universe & Multiverse | Steven

    Dear PF Forum, After searching many links in Google and threads in PF, I can't find the farthest object in the universe. I have some questions here, perhaps someone can give me quick and simple answer. A. What is the farthest object in the universe? How far away? B. The radius of the...
  32. Alex299792458

    How many cubic planck lengths are in the observable universe?

    If you take the size of the observable universe can you find out how many cubic plank lengths can fit in the observable universe and it doesn't have to be exact just approximation.Also the math and formulas would be helpful too.
  33. Drakkith

    Size of the Observable universe at the Big Bang Singularity

    Just a quick question. How big was the current observable universe at the point in time where we reach 'singularity conditions' in the early universe? I'm assuming it can't be a single point, as there is no way that I know of to make a zero-dimensional point into a 3-dimensional object or space.
  34. F

    Observable Universe question

    If we can only see things that traveled 14 billion light years away (I think) or less, then why is there a so called observable universe? If everything started at a singularity, wouldn't we have already seen everything? It's hard to put this question into words. How can something be farther away...
  35. B

    Observable universe as a black hole

    A substance of arbitrarily low density can form a black hole if there is enough of it. So I took the mean density of the universe and calculated how big it would have to be to form a black hole. It's a surprising coincidence that the swartzchild radius in light years is the exact age of the...
  36. liometopum

    Constants linked to the observable universe, not whole

    A common equation (attributable to Fred Hoyle) for the mass of the observable universe is: c3/(2GH0). Despite that the whole universe is vastly larger than the observable universe, we can create an equation to calculate the mass of the observable universe, using the standard constants of...
  37. A

    Exploring the Homogeneity of the Hot Observable Universe

    From what I understand, the observable universe began as homogeneous and very hot. if the universe was very hot, doesn't that mean that particles are vibrating at very fast speeds? after all, isn't heat simply kinetic energy of particles? if this is the case, then how could the universe be...
  38. Z

    Questions re: the edge of the observable universe

    I was thinking a bit about the farthest parts of the universe we can see and I came to a few interesting questions that are maybe obvious to more studied physicists but new to me. Is the ~14 billion light year distance we can see in all directions slowly expanding with time? In another billion...
  39. J

    How many gravitons are there in the observable universe?

    It is thought that there are approximately 10^80 protons in the observable universe, but there are approximately 10^90 photons in the observable universe. If my googling is correct, there are also approximately 10^90 neutrinos in the observable universe, but their temperature is only 1.9 degrees...
  40. F

    Neil deGrasse Tyson's Observable Universe

    In Dr. Tyson's first episode of Cosmos he said "...there are parts of the Universe that are too far away. There hasn't been enough time in 13.8 billion year history of the Universe for their light to have reached us." Since the remnants of the Big Bang has a redshift over 1000 and large...
  41. H

    Comparable size of the observable universe immediately after inflation

    Regarding scale. How big was the region of the universe we now understand to be the observable universe, immediately after early inflation? I'm tyring to understand the scale. I know our sphere of observation is very approximately 45 billion light years or something, I don't know for sure...
  42. Nugso

    The Observable Universe ve The Universe

    Hello everyone. As I was reading an article on wiki, I stumbled upon this by chance; If the universe is finite but unbounded, it is also possible that the universe is smaller than the observable universe. In this case, what we take to be very distant galaxies may actually be duplicate images of...
  43. B

    Exploring the Observable Universe: Discovering New Stars in the Night Sky

    Hello, i'm reading through a book called "Discovering The Universe" and in the chapter about cosmology the familiar idea of the observable universe is discussed. It speaks of the radius of the cosmic particle horizon being ~15 billion light years, which makes perfect sense as light from...
  44. K

    Charge balance in observable universe

    Hi, I've been wondering about the charge balance in the known universe, factors that might alter it, and the consequences of any small imbalance that might exist. This is a sort of layman's theory. I don't expect it to be right but I'd love to understand how it can be refuted. I am surely...
  45. M

    Question about the Planck spaces and observable universe, .

    From Wikipedia: http://en.wikipedia.org/wiki/Googolplex A Planck space has a volume of a Planck length cubed, which is the smallest measurable volume, at approximately 4.222×10^-105 cubic meters = 4.222×10^-99 cubic cm. Thus 2.5 cubic cm contain about a googol Planck spaces. There are only...
  46. C

    The observable universe, the actual uinverse, and CMB

    Hi all. I relish hearing from our great cosmological explainers like Neil DeGrasse Tyson and Brian Greene and watch whenever I find something new on youtube, but one thing that I don't understand and haven't heard anyone specifically address is that, if the part of the universe we are able to...
  47. julcab12

    Observable universe and events between vast distances

    Hi Guys. I would like to ask what is the very nature of event in the same moment given a vast distances. Considering Point A (earth)-vast distance-Point B (galaxies). Would be possible that the event we're observing already happened long before it reaches us and may not exist at this very...
  48. S

    Observable Universe and Black Holes

    Hi, my first post here! Galaxies outside the observable universe (that we can't see their light) can affect us with their gravity? If the answer is no, we can say that gravity information travels at the speed of light. So, in a black hole how gravity information from an object inside...
  49. G

    Measuring the size of the observable universe

    The size of the observable universe usually gets put at 93 trillion light years, though some people believe it is infinite, though I'm not sure which camp is in the majority. Those who believe it is 93 trillion light years across are they just assuming that the size of the universe at the first...
  50. B

    Tugs from Beyond the Observable Universe

    Dark Flow: Tugs from Beyond the Observable Universe? Is this true ? If true then what lies Beyond the Observable Universe ?