Big Bang, Infinity, and the Universe’s Shape Explained
The Big Bang was not an explosion from a point in space. It was the appearance of a singularity, a moment where spacetime itself breaks down and cannot be defined by Einstein’s general theory of relativity. Whether the Universe is finite or infinite in size remains an open question, since Planck 2015 data shows the observable universe is flat only within the limits of current measurement.
Table of Contents
Key Takeaways
- The observable universe has a radius of approximately 46.6 billion light-years, and its size is finite by definition.
- The Universe as a whole may be either finite (spherical or toroidal geometry) or infinite (flat or hyperbolic geometry).
- Planck 2015 data indicates the observable universe has flat geometry, within measurement limits, but this does not prove the entire Universe is infinite.
- A singularity marks a point where a mathematical quantity, such as spacetime curvature, becomes undefined, similar to how 1/x is undefined at x = 0.
- No spacetime corresponds to “zero age” after the Big Bang; every age greater than zero has a corresponding spacetime.
What Is the Observable Universe?
The observable universe is the part of the Universe that can currently be seen, and it has a finite size with a radius of approximately 46.6 billion light-years. Because light travels at a fixed speed, signals from very distant objects have not yet had time to reach us, which places a hard limit on how far we can see in any direction.
Since these signals arrive from all directions equally, the observable universe can be pictured as a sphere centered on the observer. This does not mean the observer sits at the center of the Universe itself. The observable universe and the Universe are different concepts, and the observable universe would look different from any other vantage point, such as an observer located in the Andromeda galaxy.
In this article, the Universe refers to the observable universe plus everything that lies beyond it, a distinction that matters when discussing whether the Universe is finite or infinite.
Is the Universe Finite or Infinite in Size?
Depending on its geometry, the Universe can theoretically be either finite or infinite in size. Both possibilities remain consistent with current observations.
A Finite Universe
A finite Universe could have spherical, toroidal (torus-shaped), or other closed spatial geometries. There is no “outside” from which to view this shape, since the Universe by definition encloses everything that exists; imagining it from an external vantage point is not physically meaningful.
A two-dimensional analogy makes the idea easier to grasp than a full three-dimensional topology. Picture a two-dimensional creature living on the surface of a sphere, able to move only forward-backward and left-right. If that creature travels far enough in one direction along a great circle of the expanding sphere, it eventually returns to its starting point. This is the defining feature of a finite universe: travelling far enough in a fixed direction could, in principle, bring you back to where you began, with no edge ever encountered.
An Infinite Universe
An infinite Universe would have hyperbolic or flat spatial geometry extending forever in every direction. Unlike the finite case, travelling in any direction would never return you to a previously visited location.
What Does It Mean to Say the Big Bang Was a Singularity?
The Big Bang singularity is often described as the Universe starting “from a point,” but this description is misleading. The problem lies in treating the singularity as a point located within space-time, when in fact it marks the breakdown of space-time itself.
According to Einstein’s general theory of relativity, spacetime at every event has a definite curvature. If that curvature becomes infinite everywhere, no spacetime is defined at all. Because of this, there is no time in cosmology that corresponds to the Big Bang itself; instead, the Big Bang is described as a singularity, a breakdown in the laws governing space and time.
A singularity, in general, is a point in a mathematical structure where a quantity fails to be well defined, even though it is well defined at every neighboring point. The function 1/x illustrates this clearly: for any non-zero value of x, 1/x is well defined, but as x approaches zero from the negative side the limit goes to negative infinity, while approaching from the positive side it goes to positive infinity.
Because the two one-sided limits disagree, mathematicians describe 1/x as having a singularity at x = 0 rather than assigning it any single value. The same logic applies to the Big Bang: spacetime is well defined for any age greater than zero, whether that age is one million years or one-tenth of a second, but nothing in general relativity corresponds to an age of exactly zero. That moment of zero age is fictitious in the history of the Universe, comparable to the fictitious “point at infinity” where parallel lines appear to meet on the horizon in a perspective drawing.
How Do Finite and Infinite Universes Relate to the Big Bang?
If the Universe is infinite, it was infinite at the moment of the Big Bang and remains infinite today. If the Universe is finite, it was finite just after the Big Bang and will always remain finite, meaning that travelling far enough in one direction could eventually bring a traveler back to the starting point. In both scenarios, the Universe is possibly finite in time, and in both scenarios the Universe can expand, even if its spatial extent is infinite.
What Does Planck 2015 Data Tell Us About the Universe’s Shape?
The Planck 2015 results on background geometry and topology show that the observable universe has flat geometry, within the limits of current measurement. A flat geometry is normally associated with an infinite universe, but this is not a certainty.
A two-dimensional analogy shows why the flat result does not prove infinite size. Imagine living within an infinitesimally small portion of the surface of a large sphere; measuring the curvature of that small patch would return a value of zero, suggesting a flat, infinite universe, even though the sphere as a whole is finite. In this sense, curvature measurements of the observable universe cannot by themselves determine the size or topology of the full Universe. The safest statement is that the observable universe appears flat within our current ability to measure it, while the true shape of the entire Universe, finite or infinite, remains unresolved.
Conclusion
The Universe may be finite in time, and current measurements are consistent with either an infinite universe or a finite universe whose curvature is too small to detect with present instruments. The Universe expanded from a singularity, not an explosion from a point in space, in the event known as the Big Bang, and it evolved over time into the Universe observed today.
Frequently Asked Questions
Did the Big Bang start from a single point in space?
No. The Big Bang is better described as a singularity, a breakdown in the laws of space and time, rather than an explosion originating from a specific point within space. Treating it as a point-type explosion misrepresents what general relativity actually describes.
How large is the observable universe?
The observable universe has a radius of approximately 46.6 billion light-years. This limit exists because light from more distant objects has not yet had time to reach us since the Universe began.
Is the observable universe the same as the whole Universe?
No. The observable universe is the portion currently visible to a given observer, while the Universe includes everything beyond that, whether finite or infinite. Different observers, such as one located in the Andromeda galaxy, would have different observable universes.
Does flat geometry mean the Universe is infinite?
Not necessarily. Planck 2015 data shows the observable universe is flat within measurement limits, which is normally associated with an infinite universe, but a small, curved patch of a much larger finite universe could also measure as flat.
What is a singularity in physics?
A singularity is a point in a mathematical structure where a quantity fails to be well defined, even though it is well defined at every neighboring point. The function 1/x at x = 0 is a simple example, since it approaches negative infinity from one side and positive infinity from the other.
Can the Universe be finite and still expand?
Yes. Whether the Universe is finite or infinite in spatial extent, it can still expand over time. A finite universe with toroidal or spherical geometry can grow in size while remaining finite, just as an infinite universe can expand while remaining infinite.
I am an undergraduate physics student at METU.
No, it is not an "interpretation" of what pop science says, it is EXACTLY what pop science says.Not in every case. I searched YouTube for "astronomy big bang theory" (the "astronomy" is there to avoid hits about the sitcom).
The top hit was , Phil Plait in his Crash Course Astronomy #42. I think he does a good job of acknowledging and addressing the "explosion" misconception about the Big Bang. From about 04:30 to 05:15 he talks about the universe starting as an "über-dense thing". He talks about when everything was in "one place" and says "when the universe was a tiny dot". Plait has sound credentials and probably a good grasp of the Big Bang Theory. He never says the universe started out as a point. I've played that video for high school astronomy students, though, who hear his words an meaning that.
The next hit was Crash Course in Big History. It describes the start as "many, many many times smaller than the nucleus of an atom".
The Beginning of Everything does not say anything about the initial size.
Into the Universe with Stephen Hawking does say rewinding time brings everything to "a single point", but also later says "smaller than an atom" and "very tiny"
The History of the World in Two Hours says "smaller than an atom".
I found the article and the discussion very helpful. There is, of course, a popular misconception that the Big Bang Theory says all matter space and time was once in a point that expanded outward. (This is probably a natural interpretation when taking what is found in pop science presentationsNo, it is not an "interpretation" of what pop science says, it is EXACTLY what pop science says.
I found the article and the discussion very helpful. There is, of course, a popular misconception that the Big Bang Theory says all matter space and time was once in a point that expanded outward. (This is probably a natural interpretation when taking what is found in pop science presentations and fitting it into the notions of space and time that we grow up with.) Clarifying that is a step toward demystifying the science, and that is valuable in the current environment of doubting the validity of scientific reasoning.
Two points:
1) There is a minor typo in the article: "as being covered by of the cosmology".
2) I recognize that in writing something like this you have an audience is mind and write to the background you presume it to have. Still, I recommend either developing the idea of "co-moving coordinates" or avoiding the term.
Hey I found some new theory about big bang check it out:https://www.physicsmindboggler.com/2018/01/big-bang.html?m=1
Continue reading the Original PF Insights Post.“
It's a logical definition if you look back historically and see that the Big Bang model arose long before the idea of inflation.Well yes..you are right about that
It's understandable but kind of awkward, well thanks.It's a logical definition if you look back historically and see that the Big Bang model arose long before the idea of inflation.
No.
Yes.
What you are missing is that the thermodynamic laws are more general than the particular cases you are used to. You are used to seeing them applied to cases like a gas in a pressure vessel, where there is a clear boundary between "system" and "everything else". But that does not mean the laws are limited to those particular cases. They work for any case where you can define some kind of state space for the system and some kind of coarse graining of the state space according to thermodynamic variables. That can be done for models of the entire universe like the ones used in cosmology. There are some subtleties because of gravity/curved spacetime, but they are not insurmountable.
Wikipedia actually has a decent article on this:
https://en.wikipedia.org/wiki/Thermodynamics_of_the_universeOK, thanks for clarifying and even bigger thank for the link! I'll definitely check it out. Just to point, I was not challenging any theory that currently exists – I was merely trying to figure out what's going on and why. Once again thanks for the info and for pointing in the right direction.
if we assume the infinite universe without an "outside" then doesn't that break the thermodynamic laws?No.
Or am I perhaps missing something critical?Yes.
What you are missing is that the thermodynamic laws are more general than the particular cases you are used to. You are used to seeing them applied to cases like a gas in a pressure vessel, where there is a clear boundary between "system" and "everything else". But that does not mean the laws are limited to those particular cases. They work for any case where you can define some kind of state space for the system and some kind of coarse graining of the state space according to thermodynamic variables. That can be done for models of the entire universe like the ones used in cosmology. There are some subtleties because of gravity/curved spacetime, but they are not insurmountable.
Wikipedia actually has a decent article on this:
https://en.wikipedia.org/wiki/Thermodynamics_of_the_universe
he universe represents a system if I am not mistaken and if it is expanding then how can it do so without interaction with the surroundings? Doesn't that violate basic thermodynamic principles?There ARE NO "surroundings".
If you don't like the fact that the universe encompass everything there is, then, to quote Feynman, Go somewhere else, to another universe where the rules are simpler, philosophically more pleasing, more psychologically easy.
Great article – quite well explained. However the idea of a finite universe that envelopes everything and due to this there is no such thing as "outside" of the it bothers me somewhat. I can't quite imagine how this would work. If we take thermodynamics and imagine a perfectly insulated system that can neither gain or give both heat and work then its state will stay forever unchanged. There could be no changes in volume or even temperature. Well actually temperature changes would be possible if a combustion reaction is initiated inside, but work exchange will not be possible even under these conditions due to the rigidity of the system boundaries. So ultimately if the system is to undergo changes it is supposed to be able to exchange heat and work with its surroundings, or at least work? However if we assume the infinite universe without an "outside" then doesn't that break the thermodynamic laws? The universe represents a system if I am not mistaken and if it is expanding then how can it do so without interaction with the surroundings? Doesn't that violate basic thermodynamic principles? Or am I perhaps missing something critical? I am no physicist and I might be looking in the wrong direction, and that's why I couldn't help myself but ask.
Why the BB is always considered as an event that has actually happened ?? To me, it comes as merely the temporal limit of the theoretical spacetime in the "past" direction.You can look #7
There really are three phases in cosmology as we understand it
1) t=0 the Big Bang Singularity where we don't know WHAT was going on
2) t = one Planck Time to something like t= 10E-32 seconds — the Inflationary Period (hypothetical but likely)
3) t = the end of the Inflationary Period and onward — the time of the Big Bang TheoryI found this list quite helpful:
http://www.earlyearthcentral.com/early_universe_page.html
The Planck Era (Big Bang To 10^-43 Seconds)
The GUT Era (10^-43 To 10^-38 Seconds)
Electroweak Era (10^-38 To 10^-10 Seconds)
Inflation (10^-38 To 10^-35 Seconds)
Reheating (10^-35 to About 10^-10 Of A Second)
The Particle Era (10^-10 To 10^-3 Seconds)
Big Bang Nucleosynthesis (10^-3 Seconds to 3 Minutes)
The Nuclei Era (20 Minutes to 380 Thousand Years)
There are classes of plausible theories in which time extends infinitely far into the past. One of my favorites is
https://en.wikipedia.org/wiki/Eternal_inflationActually, eternal inflation does not extend infinitely far into the past, but only the infinite future. See work by Borde, Guth, and Vilenkin: https://arxiv.org/abs/gr-qc/0110012
initial singularityThat is only stating that before physics we know of there must have been physics we don't know.
It is not supposing a physical object which is a primary cause of everything.
Why the BB is always considered as an event that has actually happened ?? To me, it comes as merely the temporal limit of the theoretical spacetime in the "past" direction.
The BB Theory is defined as starting AFTER the Inflationary Period. I don't make the definitions, I just tell'm like they are.It's understandable but kind of awkward, well thanks.
oh wait I should have just said 2. But why exactly it's not considered as in the Big Bang theory ?The BB Theory is defined as starting AFTER the Inflationary Period. I don't make the definitions, I just tell'm like they are.
2 is not, 3 IS the Big Bang Theory. If you meant are 1 and 2 not in the BB Theory, then that is correct.oh wait I should have just said 2. But why exactly it's not considered as in the Big Bang theory ?
I made the proper changes. Hope its better now.
2 and 3 are not in the Big Bang Theory?2 is not, 3 IS the Big Bang Theory. If you meant are 1 and 2 not in the BB Theory, then that is correct.
I made the proper changes. Hope its better now.
There really are three phases in cosmology as we understand it
1) t=0 the Big Bang Singularity where we don't know WHAT was going on
2) t = one Planck Time to something like t= 10E-32 seconds — the Inflationary Period (hypothetical but likely)
3) t = the end of the Inflationary Period and onward — the time of the Big Bang Theory2 and 3 are not in the Big Bang Theory?
Well yes I see. In thise first lines I was referring to general idea but just not as a singularity. I ll make the proper changesThere really are three phases in cosmology as we understand it
1) t=0 the Big Bang Singularity where we don't know WHAT was going on
2) t = one Planck Time to something like t= 10E-32 seconds — the Inflationary Period (hypothetical but likely)
3) t = the end of the Inflationary Period and onward — the time of the Big Bang Theory
Yes, I guess it depends on what people call "THE big bang". This is also confusing from literature which, e.g., places the BB after inflation. I was referring to the initial singularity, of course.Well yes I see. In thise first lines I was referring to general idea but just not as a singularity. I ll make the proper changes
Yes, I guess it depends on what people call "THE big bang". This is also confusing from literature which, e.g., places the BB after inflation. I was referring to the initial singularity, of course.I always find it less confusing to spell that out specifically as "The Big Bang Singularity" so that there's no confusion with the BB Theory
The word ”event” has a very precise meaning in relativity. It is a point in space-time.
What is usually referred to as the standard Big Bang is actually not a priori related to the singularity. It is the expansion of the Universe from a hot dense homogeneous state. Essentially the physics we know occurred. The singularity likely only occurs if you extrapolate this to earlier times using nothing but GR.Yes, I guess it depends on what people call "THE big bang". This is also confusing from literature which, e.g., places the BB after inflation. I was referring to the initial singularity, of course.
There are classes of plausible theories in which time extends infinitely far into the past. One of my favorites is
https://en.wikipedia.org/wiki/Eternal_inflationThat's really nice, I didn't know that.
Hmm, that's a good point. But still the Big Bang model is a strong model and even we don't understand the singularity cant we say the universe is finite in time?There are classes of plausible theories in which time extends infinitely far into the past. One of my favorites is
https://en.wikipedia.org/wiki/Eternal_inflation
Hmm, that's a good point. But still the Big Bang model is a strong model and even we don't understand the singularity cant we say the universe is finite in time?No, not categorically. We don't know.
Good job.
I would also add that I believe your statement that the universe is temporally finite only applies to the Big Bang model and we KNOW that that model is incomplete because it has a singularity in the math. Since we don't know what that singularity IS, we cannot say with confidence that there was no time before it. SO … I would say "In the Big Bang model of cosmology the universe is temporally finite" rather than a categorical statement that it is.Hmm, that's a good point. But still the Big Bang model is a strong model and even we don't understand the singularity cant we say the universe is finite in time?
In other cases where the universe is infinite in time, the universe got created and destroyed repeatedly? (I remember some universe models like that). It also doesn't seem to fit our universe since it expands?
Or am I missing something ?
I thought that, the Big Bang is just not an initial singularity but also "an event" (it's really hard to explain it without using the word event) that universe emerged. Like these are bounded and cannot be separated. But yes I understand your point. Is big bang just a name for the initial singularity? But not the part of the "emerging universe"?The word ”event” has a very precise meaning in relativity. It is a point in space-time.
What is usually referred to as the standard Big Bang is actually not a priori related to the singularity. It is the expansion of the Universe from a hot dense homogeneous state. Essentially the physics we know occurred. The singularity likely only occurs if you extrapolate this to earlier times using nothing but GR.
Hi,
you say the BB can be thought of as an event, but a singularity does not belong to spacetime, the union of all events (plus metric). So that can be confusing :)I thought that, the Big Bang is just not an initial singularity but also "an event" (it's really hard to explain it without using the word event) that universe emerged. Like these are bounded and cannot be separated. But yes I understand your point. Is big bang just a name for the initial singularity? But not the part of the "emerging universe"?
Good job.
I would also add that I believe your statement that the universe is temporally finite only applies to the Big Bang model and we KNOW that that model is incomplete because it has a singularity in the math. Since we don't know what that singularity IS, we cannot say with confidence that there was no time before it. SO … I would say "In the Big Bang model of cosmology the universe is temporally finite" rather than a categorical statement that it is.
Hi,
you say the BB can be thought of as an event, but a singularity does not belong to spacetime, the union of all events (plus metric). So that can be confusing :)
In an actual finite universe, which would typically be expanding or contracting, one might have to travel faster than the speed of light in the expanding case in order for the mover to arrive at the same spacial point. It may also be useful to mention choosing the point of interest as fixed in co-moving coordinates.That's a really good point. I never thought that when I was trying to explain the concept…Thanks :)
Hi @Arman777
I like your presentation, but I feel it would be improved by mentioning the following point.
Here is a quote.
In an actual finite universe, which would typically be expanding or contracting, one might have to travel faster than the speed of light in the expanding case in order for the mover to arrive at the same spacial point. It may also be useful to mention choosing the point of interest as fixed in co-moving coordinates.
Here is a suggestion.
Regards,
Buzz