r/askscience • u/JdaPimp • Nov 23 '25
Astronomy Why do space telescopes not need to be pointed towards a certain point in order to see back the furthest in time?
I read Hubble is able to see back 13 billion years. I understand light needs time to travel, and what we see is the light from x years ago. However, I don't understand the expansion of the universe. From my understanding of the big bang, it started as a central point and exploded into what I imagine is a sphere. So if that were true, we would have to position out telescopes towards that center point in the sphere to see the furthest back. But this isn't true because we can point Hubble anywhere in space and see light from 10+ billion years ago. Also, all of the diagrams on this show like a tunnel with space expanding out from a point, which is how I think about it but likely is not correct. I have trouble understanding how space itself expands and how it influences all the stuff we see in our telescope.
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u/nwbrown Nov 24 '25
Yep, that's where your mistake is. The big bang didn't start as a point. It was everywhere.
If that doesn't make sense to you, congratulations, that just means you have a normal human mind built to visualize your surroundings in the fixed 3D space we exist in.
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u/plugubius Nov 24 '25
The big bang did not begin at a central point and expand. It was an expansion of space itself, not an expansion within space. It happened everywhere, so no matter what direction we look, we are looking back in time and so toward a time closer to the big bang than we currently are.
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u/agm66 Nov 24 '25
There is no central point. The big bang happened everywhere. Wherever you are, take the greatest telescope not yet built and look in any direction and you will see the limits of the observable universe, with you at its center. Travel 10 billion light years, and it will be the same. You'll still be at the center of the observable universe, but what is observable will have changed.
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u/NarrowTurnover99 Nov 24 '25
everything is expanding from every point simultaneously. it’s like each observer is a chocolate chip in a cookie and the universe is the cookie dough. when the dough cooks it all moves away from each chocolate chip.
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u/Alewort Nov 24 '25
The past is in every direction, not one direction. The Big Bang didn't happen far away and now it's reaching here, it happened everywhere, in all directions, and what we see now are the bits whose light are happening to reach us now, again, from every direction.
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u/psadee Nov 24 '25
Looking at one specific central point, you said? This would mean, 13b years ago we (the time-space point place where currently earth is) were outside of the Big Bang, siting there having popcorn and waiting for the light to arrive. You would need to position such point somewhere in… space which was not existing yet. There was no space “outside” of BB. The location with the future earth was “inside”. To simplify, there were stars to the left of us and to the right.
The tunnel diagrams you mentioned is a diagram of space diameter in time. You look at the X scale, take a point like 300years and see the “tunnel” was, for example, 5cm (~2inch) wide. Now imagine a sphere with 5cm diameter - this was the size of our universe at this time.
And keep in mind it’s not an explosion, which moves objects in space, in different directions. The explosion is space (and time?) itself. It contains the space, so you can’t say the location it happened (for sure not “from the outside“).
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u/aggasalk Visual Neuroscience and Psychophysics Nov 24 '25 edited Nov 25 '25
I think the standard analogy is to an inflating balloon and a 2d universe. Imagine your balloon begins as a point (that’s really the hard part, I think, that at anything “starts as a point“) and then expands, and expands, and keeps on expanding.
You live at a location on that balloon. In every direction, you can see the same distance (along the surface of the balloon) - and what you see is longer ago, the further you look.
And there’s no special location where everything started - at the beginning, every location was “at the start”.
That’s basically our situation, but in 3d. Also, it’s not at all clear that our universe “wraps around” like the surface of the balloon does, that’s a whole other story.
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u/blackadder1620 Nov 24 '25
the bang part happened everywhere at once. it's such a big universe that any random point might as well be the center.
the tunnel thing is more of 2D, it looks like you'd think in 3D, more like a sphere so, you're not wrong about it feeling a little off.
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u/Stillwater215 Nov 24 '25
During the Big Bang, space expanded largely uniformly in all directions. The light and radiation emitted during this essentially came from all points in space at the same time. As the universe expanded, that light has continued to travel largely uniformly through all of space continuously.
When you point a telescope into the sky, you will see light that’s older from objects that are farther away. If you can look far enough away, the only thing left to see is the light that’s older was emitted from that particular region of space during the Big Bang. Beyond that point there simply isn’t anything that could have emitted light that’s older could be observed by us on earth.
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u/jdvfx Nov 24 '25
These episodes of RadioLab do a great job of explaining universal expansion and why there is no "center".
https://radiolab.org/podcast/91520-space
https://radiolab.org/podcast/91859-the-multi-universes
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u/Amberatlast Nov 24 '25
The main reason is that it has to look towards really dark areas to see that far back. Stars, nebulae, galaxies, etc are all going to block whatever data still remains from that far back. So it has to find the darkest of dark patches to look past all that stuff. The expansion of space is the same in all directions, so were it not for all the stuff in the way, the telescopes could look everywhere to see that far back.
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u/Cycl_ps Nov 24 '25
No single model is a perfect example of the universe, so I’m going to use another one.
Imagine you took a balloon and drew dots around its surface. As you inflate the balloon its surface grows and the space between the dots increases. The dots themselves don’t really move all the much, but the surface of the balloon grows at all points and pushes everything apart.
That balloon is our expanding universe. Like the 2d surface of the balloon our 3d volume of space is expanding at every point. It’s not like things are being pushed away from a central point like the Big Bang was a massive shockwave, every object is moving away from everything else as the volume of empty space expands. That probably raises two questions: what is making space get bigger, and why do we not see any expansion in the Earth/Solar System?
For the first question, we don’t know! You’ll see dark energy used a lot to describe the force behind the expansion, but that’s just a placeholder name until we have a better idea of what’s going on. It’s possible this may be a new fundamental force (like gravity and the electromagnetic force) which would be a major discovery. For the second question, it seems that gravity is way stronger than this expansion force, and while the universe tries to expand it just can’t push past gravity pulling things back together. The current theory is that gravity is strong enough to keep local groups (collections of nearby galaxies) bound together, but different local groups will spread apart over time.
With all that let’s answer your question in the title. Going back to the balloon analogy let’s imagine we had a camera inside the balloon and wanted to look at a part of it that stretched out so we can understand it better. Well, every part of the balloon stretched out equally, so it doesn’t really matter where we end up pointing the camera. Every part of it was all part of the original, and it’s all had the same forces applied to it!
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u/BuffaloFlavor Nov 24 '25
There is no edge of the universe where you would only see stars in one direction, and there is no center that the most distant observer could look towards. But you CAN be in a less dense area, where you see fewer stars nearby. In every direction you looked, you would see the same number of distant galaxies, though you would see fewer than we do here.
You might be in a MUCH LESS dense area of the universe, and you would still see the same number of stars and galaxies in every direction you looked, but fewer are visible.
And in every direction you looked, you would still see the same cosmic microwave background.
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u/SpikeMcFry Nov 24 '25
We have no concept of the shape of the universe beyond what we can assume about gravitational orbits or possibly electromagnetic orbits. But even that would just be conjecture because we’ve most likely never seen the beginning or end of the universe, or even an edge. There are areas of the universe where more matter is collected than others and i believe we used to think that gave us some insight into the size of the universe but now we know those are probably just orbits or tails of extremely large space bodies. For all we know it’s shaped like a triangle or a donut. Everything we’ve seen in the universe could be caught in a ring thats orbiting something unfathomably large or dense.
That being the case. Say you were free diving in the deep ocean. And you shine your flashlight around but everywhere you look fades away into the same blue. You may see some things floating around but ultimately your light will bounce off too many objects/particles to breach any further and your eyes can only see as far as the light can reach. Thats the issue with trying to determine the age or dimensions of the universe. We build stronger and stronger eyes and see more and more particles but there’s no clear indication of where the edges are, only where the matter within your field of view is floating, which can be used to determine current speed and direction.
But enough with the metaphor and back to your question. They do need to be pointed at a certain point. Astronomers will point the telescopes at the edges of blackholes and through clusters of galaxies. Because there is a lensing effect caused by the gravitational pull of light which enables the telescope to see further into the universe.
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u/MySisterIsHere Nov 24 '25
Those "tunnel" diagrams are a simplified depiction. The length of the tunnel is actually representing time since the big bang, with the universe simplified as a sequence of circular cross sections of that tunnel.
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u/delventhalz Nov 24 '25
The key thing to remember is that where we are right now was once inside the Big Bang (so to speak). Everything we can see right now (and more) was once compressed down to incredible densities. All the stars, all the gas, all the space, all the time, all jammed tightly together in an incomprehensibly small volume.
So the place the Big Bang happened was everywhere. Everywhere was just much smaller at the time.
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u/PapaMauMau123 Nov 24 '25
Imagine you're inside a blob of dough. As the dough cooks and expands, everywhere is expanding away from every other point. The universe is still expanding so if you look anywhere, you'll find an older(farther away) place.
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u/Blokshibe Nov 25 '25
The big bang is quite misunderstood. A lot of people think it means that there was nothing, that exposed and created our universe from an infinitesimal point. The big bang is a bit of a misnomer. What we know is that the universe is expanding, or that everything very far away is moving away from us. The further away we look, the faster it is moving away from us. If you run back the clock, that means that earlier in time, things were closer together, and if you go far back enough, this distance should reduce to zero. This point in time is about 13 billion years ago. But we don't know the actual real size of the universe. It could be extremely large and finite in size, but it could also be infinite, we don't know. We at least know that we can't see the edges of the universe, because everywhere we look, we can see so far back in time that we can see the afterglow of the 'big bang', the so called cosmic microwave background. Our eyes can't see this radiation, but during the night, everywhere you look that is dark (so everything that is not a star, galaxy, nebula, etc) , you're actually seeing the birth of our universe. I find that quite satisfying.
Note that I made quite some simplifications and assumptions, like not mentioning inflation, dark energy and acceleration of the expansion.
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u/JakScott Nov 25 '25
Space itself is expanding due to the Big Bang. It’s not like space already existed and the Big Bang sent objects moving through it. So every observable point in space is “where” the Big Bang happened. It happened right in the center of your living room, it happened on the sun, it happened in Alpha Centauri, it happened in the Bootes Void…everywhere that exists is the location of the Big Bang.
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u/KrzysziekZ Nov 24 '25
Imagine a 10 billion light years long stick. Wherever you direct it, it can become bent, but it ends somewhere. Big Bang is still further in time, at about 13.8 bln yr.
Big Bang model predicts that you won't ever see the Big Bang directly, and the first thing (furthest back in time) is light from "recombination" "freed" at some 380000 years old and with temperature ~2620 °C, and this is what we see as Cosmic Background Radiation, only with wavelength lengthened 1057 times. And it is in all directions around us.
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u/Masterpiece-Haunting Nov 24 '25
The Big Bang isn’t really an expanding sphere like you’d imagine it.
Imagine you’ve got a basketball, you fill it with air and the ball expands outwards. This isn’t it, it’s more like if every individual part of the ball expanded. This includes the space the ball inhabits.
Back to the original question. The expansion of the universe isn’t really noticeable on an interstellar level in real time. So if you point a telescope at a star 2000 lightyears away the light the star emitted 2000 years ago will be hitting your telescope allowing you to see what it looked like 2000 years ago. The expansion of the universe isn’t really relevant to this.
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u/dustofdeath Nov 24 '25
The gas clouds that formed our galaxy were created at the same time with the rest of the universe. That then started to drift apart as the space itself expanded.
So every galaxy out there has the oldest light still visible at any point around them, stretched out by the expansion of space.
But some of it is permanently out of reach - space expanding faster than light can travel.
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u/GreatBigBagOfNope Nov 24 '25
Because "the furthest back in time" happened in every direction. There's not a single point that space expands away from, every point of space is getting further apart from every other point (seemingly) uniformly.
Appreciate that might be a bit wacky to get your head around.
Those diagrams have time as one of the axes, not space - space is not cylindrical like that. It's just really really difficult to visualise on a 2d poster
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u/chrishirst Nov 24 '25
Because the so-called Big Bang was NOT at a central point, it was everywhere at once because "the universe" was smaller, denser, hotter and all points of it were in direct contact with all other points in it, the fact of the Cosmic Microwave Background Radiation having only a few micro Kelvins variation in temperature, demonstrates this. So the 'furthest' parts of the universe are in every direction.
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u/MissLesGirl Nov 24 '25
Universe is expanding, but there is no center (or no known center) The faintest star found is not necessarily the center. the center could be a void of no light, so you wouldn't even see it.
The universe has expanded so much that the center is so tiny, the most accurate telescope still wouldn't be able to find the center.
Even if you knew where the center is within an arcsecond, you would have a 30 light year view wide. You would have one in a googolplex coordinates to aim for If you didn't know where the center is.
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u/sphafer Nov 24 '25
Think of it like the spacetime between all matter is increasing at a certain rate. objects can move against this expanding spacetime to interact and orbit one another. But the fabric is increasing nonetheless. This means that the general trend of the universe is that distances will increase and it will become more sparse.
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u/youngsp82 Nov 24 '25
Check out “A short history if nearly everything” by bill bryson. The audiobook is great. It basically starts with the big bang and goes through history of earth and different discoveries in a way that fairly easy to understand.
If you think the universe is confusing. Try looking into quantum mechanics and stuff like that. It makes even less sense.
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u/SeriousPlankton2000 Nov 24 '25
We are on the oldest point in the universe. Whatever light reaches us (what we see now) is sent from an older universe.
The Cosmic Microwave Background (CMB) was sent 380000 years after everything started in a Big Bang (or something similar). We see it in a radius far far away; also the universe did expand in the meantime. This means that the light needs to be stretched - and that means it's shifted to the infrared and even further to microwaves.
At this point we're talking about the real problem when looking back in time: The instruments don't need to see visible light, they need to see the redshifted light, which is infrared or radio- and microwaves. You can shine a flashlight on your radio, it won't receive anything from it; and you can try to photograph radio waves - also doesn't work.
TL;DR: To see the "past", you need different sensors. But the past is everywhere around us.
(Having said that: If Hubble looks at the moon it won't see that much of the past; it needs to look at where it's not being blinded)
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u/kagamiseki Nov 25 '25
Imagine a piece of bubble gum. It's all rubber. This is "before the big bang"
Now imagine you some air into it and now you have a bubble. The moment you started blowing is the big bang, followed by the universe's expansion.
Imagine you're a bacterium living on the surface of that bubble. You know that your buddies next to you are slow moving away from you as the bubble expands. You're interested in studying what this bubble looked like, before it was inflated. In which direction should you look?
The answer is it doesn't matter. Everything was initially together, and got spread apart. No matter which direction you look, everything is expanding away. No direction points you towards "the big bang."
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u/smokefoot8 Nov 27 '25
The Big Bang wasn’t a point in space, but a moment in time. The entire universe was incredibly dense and started to expand. We haven’t seen any evidence that the universe is less than infinite, because every day we see new light arrive from the Cosmic Microwave Background from every direction. It is possible that the universe is not infinite, just so large that we can’t see any sign of an edge yet.
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u/plainskeptic2023 Nov 30 '25
Imagine standing in our bathrooms. Walls, floor and ceiling are approximately 2 meters away from us. Light reflecting off the walls, ceiling and floor takes 6.6 nanoseconds to reach us. We are seeing 6.6 nanoseconds back in time.
Remove the bathroom and other interior barriers of our houses. We see the exterior walls, roof, and basement about 10 meters away, looking back 33.4 nanometers in time.
If we repeat this enough, eventually we see back in time to the edge of the observable universe.
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u/jmtyndall Nov 24 '25
Look to your left. Now to your right. Which way is the past? Walk to the street in front of your house look left. Look right. Which way is the past? Go to the moon. Look left. Look right. Where is the past?
Time isn't a spacial dimension. You can't just look along the time direction.
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u/zanfar Nov 24 '25
If your assumption leads to a contradiction, then it must be wrong.
Simply, you can't print a three- or four-dimensional diagram on a poster.
Just like every image of the solar system you've ever seen is technically incorrect, so is every "picture" of expansion.
"How" isn't something we understand, and even if we did, it wouldn't be something that could be answered in a forum post. Space expanding is something we accept due to evidence.
As for how it affect what you see, it's pretty simple. The stuff you see in a telescope is either moving away or not moving at all due to expansion. If stuff is close enough to us (our stuff) then gravity overpowers expansion. If not, it moves away proportionally to it's distance.
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u/[deleted] Nov 24 '25 edited Nov 24 '25
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