# How big Is the universe?

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Google has told me that the universe is 93 billion lightyears in diameter and I was also taught it was expanding at the speed of light. But I was also taught it was 13.7 billion years old meaning it must be at least around 27 billion light years in diameter so is you universe 93 billion years old  or is it 27 billion light years in diameter. What do you guys think?

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Posted (edited)

First off, you mean "observable universe" i.e. what we can actually see, in theory at least. The universe is thought to be infinite.

The reason it's 93 billion ly (or whatever the currrent estimate is) is because the universe is expanding and has been all throughout time. So things that were far away are now even further - but when light left them they were near enough to be within the "observable" distance.

Edited by wulfrun
units corrected
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2 hours ago, VirgoCluster25 said:

I was also taught it was expanding at the speed of light.

Where were you taught this? This isn't correct; the universe is, in some sense, expanding faster than the speed of light 😁. The expansion of the universe is governed by general relativity, and general relativity can be munch more non-intuitive than special relativity!

We are "over here", while distant galaxies are way "over there". Because of spacetime curvature between "over there" and "over here", it is difficult to define the speed of an object "over there" with respect to us "over here" in a way that respects all of our everyday experiences with speed. This leads to a first explanation for the possibility of recessional speeds greater than the speed of light.

Special relativity prohibits speeds greater than the speed of light. Cosmology, however, is governed by the curved spacetime of general relativity, to which special relativity is a good *local* approximation. Consequently, we will never see anything moving faster than the speed of light in our local neighbourhood, where special relativity is a good approximation. Stuff at the edge of the universe is not in our local neighbourhood, and thus is not governed by the laws of special relativity.

Alternate (more technical) explanation for recessional speeds greater than the speed of light.

speed = distance/time, so if different definitions of distance and time are available, we can have have differing definitions of speed. The definitions of distance and time used in cosmology lead to cosmological recessional velocities that correspond not to velocity in special relativity, but to something different called rapidity (sometimes called the "velocity parameter"). In special relativity, there is a relationship between velocity and rapidity, which, for some reason is not used in cosmology. If this relationship were used in cosmology than a recession rapidity of 3.4 corresponds to a recessional speed of 0.998 times the speed of light.

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6 hours ago, VirgoCluster25 said:

Google has told me that the universe is 93 billion lightyears in diameter and I was also taught it was expanding at the speed of light. But I was also taught it was 13.7 billion years old meaning it must be at least around 27 billion light years in diameter so is you universe 93 billion years old  or is it 27 billion light years in diameter. What do you guys think?

Depends on how you measure.

If we had ruler that is big enough and we could stretch it to the furthest parts of universe that we see now - that ruler would be ~46.5 Bly long.

If we time the light that started back in the past from that point that we consider "edge" - it has taken ~13.8 By to reach us - so we would conclude that edge is ~13.8 Bly away from us.

How come? We know that speed / length and time are related by simple formula, right? speed = distance / time. Simple, right?

Well - yes if you have space and time that don't stretch and bend.

Our universe is expanding and above formula does not hold.

Light indeed traveled form 13.8 Bly to reach us - but while it was traveling, space "in front" and "behind it" was also expanding. Space "in front" contributed to how much light needed to cross, but space "behind" the light does not contribute to length measured by speed of light and time it took it to travel it - yet, that space expanded and if we fit ruler "now" - it would show above ~46.5 Bly to the place where light first started.

This is by the way - size of observable universe. Not the whole universe. Just the part that we see.

We simply can't see further than that because light from those parts did not reach us yet. There are parts of universe that we will never see - no matter how long we wait for light to reach us - simply because universe is expanding faster than light can catch up.

By the way - universe is not expanding faster than light - it is just that there is so much space between here and where light is - that universe expanding slowly locally builds up over such a large distances - that it expands more than light manages to cover for a given time period - and there is always more distance to be covered then it was on the start of the journey.

We do know that universe is at least 1000 times larger than what we can observe. This is our current limit on measurement of flatness of the universe. Universe can have certain geometry to it - positively curved, negatively curved or flat. We are measuring that it is flat - but the problem is - we can only improve our measurement - and never really fully measure that it is flat (there is always margin of error and no matter how precisely we measure it - there is still possibility that it is curved but with curvature less than our margin of error).

In any case - given our current measurement we know that universe is between infinite and x1000 larger than what is the size of observable universe. So yea, huge

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I will just say it is big beyond our ability to comprehend.

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if the universe is expadning slower then the speed of light acording to you then it should be smaller then 13.7 bly. And virgo cluster being 5 million light years in diamiter if that and the universe is flat completly it would take up 0.005% of the universe. {Correct me if my maths was wrong} if the universe was 93 bil. if its 13.7 bil then it could take up 0.05% of the universe {if they where completly flat and the last one is not to scale and is probably wrong but it was a guess}. But i know alot of you will say that it is expanding at the speed of light and it cannot take up a percentage of something thats expanding. But it would take 1 billion years until it becomes 94 billion years. So if it stopped expanding then virgo cluster being 5 million years in diamiter should take up the percentage said. But why am i telling you about this virgo cluster stuff. Well if currently the universe is straveling slower then light its less then 27.4billion light years diamiter {Because its 13.7 light years from the centre expanding from all side} then its scientificaly less then 13.7 billion light years making virgo cluster insanley large.

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On 17/04/2023 at 13:11, wulfrun said:

First off, you mean "observable universe" i.e. what we can actually see, in theory at least. The universe is thought to be infinite.

The reason it's 93 billion ly (or whatever the currrent estimate is) is because the universe is expanding and has been all throughout time. So things that were far away are now even further - but when light left them they were near enough to be within the "observable" distance.

if the universe is already infinate then is the observable universe expanding at the speed of light or do you mean that you universe itself is infinate because its infinatly expanding? Because if its because its infinatly expanding then i mean at the moment its 93 billion ly in diamiter. Its not exact but its estimated.

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4 minutes ago, VirgoCluster25 said:

if the universe is expadning slower then the speed of light acording to you then it should be smaller then 13.7 bly

Depends how you define speed of expansion.

Universe is not two distant points that move away from each other at some speed and that is constant. It does not work like that. Best way to think about it is this - picture universe as a grid:

and have any two neighboring points move away from each other with some velocity.

I've marked few pairs of points - and they all move away from one another with the same velocity. However, if you measure recession speed of these pairs:

you will find that they move from each other at twice above speed. Simply put in above image, if we look at top pair - left one is moving away from its first neighbor with some recession speed, and right one is also moving away from its first neighbors with that speed - so they move from each other with twice that speed (as speeds locally add "normally").

At current time this speed is H0 or Hubble constant at current time and it is measured to be around 70 km/s/Mp - or 70 km per second per every mega parsec of distance.

In above image if two neighboring points were 1 mega parsec apart - they would move away from each other at 70 km per second. Then points that are two mega parsecs away - would move 140 km / s and so on.

Important bit to remember is that we don't have our "warp engine" on - and we are not actually moving thru the space away from every other point. It is space between us that is stretching. We are actually not moving away at high speeds from distant objects in sense of special relativity. Otherwise - we would measure those distant objects to have enormous mass - close to infinity (and that would cause all host of issues). It is the space that is stretching.

20 minutes ago, VirgoCluster25 said:

And virgo cluster being 5 million light years in diamiter if that and the universe is flat completly it would take up 0.005% of the universe. {Correct me if my maths was wrong} if the universe was 93 bil.

Depends how you define taking up.

Observable universe is 93000 Mly in diameter and Virgo cluster is 5 Mly in diameter. That is 93000 / 5 = x18600 times smaller in diameter. But if we want to know about "taking up" - we need to think in volume and not diameter. We can simply switch from diameter to volume by raising things to third power. 18600^3 = 6434856000000

Virgo cluster takes up 1/6434856000000 of volume of observable universe (as measured by ruler).

24 minutes ago, VirgoCluster25 said:

f its 13.7 bil then it could take up 0.05% of the universe {if they where completly flat and the last one is not to scale and is probably wrong but it was a guess}.

You can't compare these two units as those are not comparable quantities. 13.7 Bly means that light took that much time to travel certain distance. As I've explained - light traveled while space was stretching and amount of what light traveled does not reflect current spatial extent of observable universe.

Imagine that you run on a train. Train is 1km long, but it travels really fast between two major cities - distance of 50km by the time you get from one end to the other end of train running.

If we look at your fitness watch - it will tell us that you only ran for 1km, but you managed to cover distance of 50km. If you did not know about the train - you'd think you ran from London to Luton in dozen of minutes

This is the same thing with light - it thinks it traveled 13.7Bly, but in reality it traveled for 46.5Bly because it was on a train (or rather inside of expanding universe).

For that reason - we can't take 13.7Bly as realistic if we want to compare volumes, as it is distance that is combined with view backward in time (further we "look" - more back in time we see).

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29 minutes ago, VirgoCluster25 said:

if the universe is already infinate then is the observable universe expanding at the speed of light or do you mean that you universe itself is infinate because its infinatly expanding? Because if its because its infinatly expanding then i mean at the moment its 93 billion ly in diamiter. Its not exact but its estimated.

No - size of universe and rate of expansion are not directly related. Don't think of rate of expansion as speed at which "ends" of universe move away. Think of rate of expansion as speed at which two close points move away from each other (or to be more precise - any two points in universe that are separated by that distance).

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28 minutes ago, VirgoCluster25 said:

if the universe is expadning slower then the speed of light acording to you then it should be smaller then 13.7 bly. And virgo cluster being 5 million light years in diamiter if that and the universe is flat completly it would take up 0.005% of the universe. {Correct me if my maths was wrong} if the universe was 93 bil. if its 13.7 bil then it could take up 0.05% of the universe {if they where completly flat and the last one is not to scale and is probably wrong but it was a guess}. But i know alot of you will say that it is expanding at the speed of light and it cannot take up a percentage of something thats expanding. But it would take 1 billion years until it becomes 94 billion years. So if it stopped expanding then virgo cluster being 5 million years in diamiter should take up the percentage said. But why am i telling you about this virgo cluster stuff. Well if currently the universe is straveling slower then light its less then 27.4billion light years diamiter {Because its 13.7 light years from the centre expanding from all side} then its scientificaly less then 13.7 billion light years making virgo cluster insanley large.

You have to be careful with the phrase 'expanding faster/slower than the speed of light.'  The expansion of the universe, outside gravitationally bound systems, must be measured on a cubic unit by cubic unit basis. Small cubic units of universe don't expand by much at all. It is only when you have an enormous number of cubic units, each expanding by a small amount, that you end up with distant objects moving away from each other at huge speeds.

Olly

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"Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space." Douglas Adams.

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I had always thought that the Universe was tiny, smaller than a photon or electron...

Alan

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Posted (edited)
4 hours ago, VirgoCluster25 said:

if the universe is already infinate then is the observable universe expanding at the speed of light or do you mean that you universe itself is infinate because its infinatly expanding? Because if its because its infinatly expanding then i mean at the moment its 93 billion ly in diamiter. Its not exact but its estimated.

If the Universe is spatially infinite then it always was infinite and this is the case in the currently accepted concordance LCMD model.

If you think of @vlaiv grid then it always extended to infinity. What is changing is the distance between the dots. This is called  metrical expansion.

Regards Andrew

Edited by andrew s
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Posted (edited)
3 hours ago, Alien 13 said:

I had always thought that the Universe was tiny, smaller than a photon or electron...

Alan

I like this from William Blake:

"To see a World in a Grain of Sand And a Heaven in a Wild Flower, Hold Infinity in the palm of your hand And Eternity in an hour."

Regards Andrew

Edited by andrew s
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15 minutes ago, andrew s said:

I like this from William Blake:

"To see a World in a Grain of Sand And a Heaven in a Wild Flower, Hold Infinity in the palm of your hand And Eternity in an hour."

Regards Andrew

One of my favourites.

Jim

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Posted (edited)

Andrew's extract from Blake's "Auguries of Innocencegot me reflecting on a related (tangent) theme from a favourite 1950 science fiction B movie "The Incredible Shrinking Man".  The central character in the movie is caught in a mysterious mist which, over the following weeks, causes him to shrink in size. In the closing scene, he has shrunk to such a diminutive size that he is lost from the sight of his family.  Alone in the basement of his house, having fought of the resident spiders and assorted bugs, he continues to shrink even further, presumably destined to enter the realm of microbes and then beyond.  Having now lost everything, with an uncertain future ahead, he questions his own humanity. In the very final scene, his view turns skyward through a window in the cellar and his gaze falls on the night sky and the stars in the heavens above.  For all that his future places him beyond humanity, the life and world that he once knew, he considers how the view of the night sky and his relation to the stars above is the only thing that has remained unchanged to him.  The scale of the universe is as infinite to him now and will be in his future as it was before.   For me that is what Blake means when he says "to see the world in a grain of sand, to hold infinity in the palm of your hand ".  To answer the OP's question then; the numbers we place on the size of the universe are from a practical view largely irrelevant, its scale is lost to our common understanding.

Jim

Edited by saac
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On 17/04/2023 at 12:51, VirgoCluster25 said:

Google has told me that the universe is 93 billion lightyears in diameter and I was also taught it was expanding at the speed of light. But I was also taught it was 13.7 billion years old meaning it must be at least around 27 billion light years in diameter so is you universe 93 billion years old  or is it 27 billion light years in diameter. What do you guys think?

I think we've been measuring it wrong.  If the outer universe we see is 13.7 billion years old, then during that 13.7 billion years if it expands at the speed of light what would it be now?  Maybe 93 billion and 13.7 billion are both correct answers.

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??? How long is a ball of string???

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It’s big, but whether it’s infinite or not the jury's out on that one. Even scientist’s can’t agree on that question, so what chance have we mere mortals on SGL?

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Consider a train travelling on a straight line away from you at 40mph.

When it's 200 miles away it releases a pigeon that flies back to you at 50mph.

It takes 4 hours for the pigeon to get its message you.

By which time, the train is 360 miles away.

The expansion of the universe is not like this, but it sort of gives an understanding of how something that emitted light 13By ago can be 47Bly away.

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The train analogy is a little helpful to a befuddled father who is having to explain to his young son with a growing interest in astronomy that

(1) nothing can travel faster than light

(2) the universe is 13.8 billion years old

(3) the universe is at least 90 billion light years from one side to the other

(4) that as Carl Sagan used to say the universe is more weird than we can imagine.

I always thought intergalactic space was a vacuum, with the occasional stray hydrogen molecule showing up. In which case what is expanding? If it's actually more like a lattice, in which the poles connecting the junctions are getting longer at faster than the speed of light, then what are the "poles" made of? And is only intergalactic space expanding? is interstellar space also getting bigger? For that matter is the solar system expanding? Are the atoms that make up my body bigger now than they would have been when the earth came into existence 5 billion years ago? Or are they further apart? If not, then suppose I created a rope so long it could stretch from one galaxy to another. As the galaxies fly apart will the rope get longer? Or will the nearest galaxy to one end get further away from the end of the rope over time?

My brain hurts.

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The expansion of the Universe does indeed result in a tension pulling things apart.  However,  it is orders of magnitude weaker than the electromagnetic force holding atoms and rulers and other objects together.

Similarly,  gravitational bound systems like the solar system and cluters of galaxies are bound by orders of magnitude more strongly than the expansion induced tension.

It's only when object are much much further apart that the tension is greater than the gravitational attraction and they move apart with the expansion.

Regards Andrew

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1 hour ago, Starfazed said:

In which case what is expanding?

Problem is that we are used to understanding things by means of analogy.

Most of the time this works well, but sometimes it fails - and in those moments we are very confused and come up with phrases like - "no one can understand quantum mechanics", or questions "how can empty space be expanding".

Here is another question that you probably never thought about - but is similarly "weird" when you think deeply about it. Why do two electrons repel each other? Or another one - why do some particles have property called spin? What is spinning? Are those particles little balls that are spinning?

See there, I just tried to use analogy to things that I know with electron spin, but simple fact is - it is property of electron that is sort of intrinsic - it is something that we observe and don't necessarily need to explain it in everyday terms.

Similarly - light travels at fixed speed. Weird? Yes, but only because we are used to other things traveling at different speeds and behaving differently - and only when we try to apply analogy to light - is when we get confused.

You never thought about it - and never had a chance to see it - but empty space is simply expanding. That is property of empty space (much like electrons repelling each other or having spin, or speed of light being fixed).

From General Relativity we know that energy and matter bend space and time. Well, it turns out that space "bends the other way" when empty.

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• 2 months later...
Posted (edited)

Thanks, @vlaiv et al. Or, if you prefer a VIDEO on this:
https://youtu.be/u23vZsJbrjE (ITMA: Dr. Lincoln, FermiLab)
Always good to re-visit things, whatever I think/thought? 😁

The size of the entire universe depends on its geometry? 😉
MIcrowave background spot-size, suggests, "space is flat".
But curvature uncertainty, allows closed, open hyperbolic...

Flat or Open, are consistent with space being of *infinite*
size... But a closed universe can be no smaller than 250x the
visible universe? Probably it's the *limit*, one should quote?
Ideally, you should check, whether it's Diam or Radius? 😛

Edited by Macavity

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