How Do We Know the Universe is Flat?
One of the things you hear a lot in cosmology, particularly when discussing the amount of dark energy there must be, is that the geometry of the universe tells us a lot about what’s in it.
For example, in order to have a flat, accelerating universe, 70 percent of our universe must contain something like dark energy. Anything other than that, less or more than 70 percent, and we have another shape.
This begs the question, how do we know the universe is flat? So much of our inventory of the universe depends on satisfying this condition.
It’s actually pretty clever. As with any geometry project, we need triangles. If we take an enormous triangle and add up all the angles, we should get 180 degrees if the universe is flat, less than that if it’s negatively curved (middle surface in diagram above) and more than 180 degrees if its positively curved (the sphere at top).
Now, all we need is a triangle. A big one.
It turns out that the Cosmic Microwave Background provides a great triangle. Using the Earth as one apex, and measuring the largest fluctuations in the CMB, if the universe is flat, those fluctuations should be 1 degree across. A negatively curved universe would give smaller angular sizes, and a positively curved universe would yield larger ones.
It turns out that the largest fluctuations measured in the CMB from the WMAP satellite are, in fact, 1 degree across.
Who said cosmology was hard?
Blue areas are cool regions, yellow and red are slightly hotter. Largest angular size of fluctuations = 1 degree
BTW, this is brilliantly explained in Sean Carroll’s Cosmology Course. If you are even a little interested in this stuff, it’s well worth the money.
Technorati Tags: cosmic microwave background, cosmology, dark energy, dark matter
Tony, I can’t quite wrap my head around this flat concept, maybe you can help me… I understand the geometry of using triangulation, but the goedesic seems to make more sense. Here’s how I see it:
If I can go outside my house and look straight up somewhere in the area where I look, almost infinitely beyond the distance I can see is the ‘edge’ of the universe (which is expanding). Then if I went to another location and looked straight up, the expanding edge of the universe is somewhere up there, too. Pick any point on this Earth and repeat the process, you should have the same result, right? And since the Earth is more or less spherical in shape, couldn’t we apply that logic to mean the universe is moderately spherical in shape as well? Wouldn’t the Hubble Deep Field find the same general result at approximately the same distance and exposure, regardless of which area of the universe it peered into? I just can’t see the flat in that.
By saying it’s flat, your suggesting that two points on Earth, at any given time, are much closer to the edge of the universe than other. By flat do you mean more like a shoebox? How thick is flat? This concept seems to suggest that the closest edges of the universe might right now be hovering just above Poughkeepsie and it’s global antipode, while at the equatorial ring for the axis of those two points, the universe extends drastically beyond – shaped more like a CD or LP record.
Shouldn’t then the Hubble be able to see the closer edges of the universe, given that it’s pointed in the right direction – the flat top and bottom, so to speak? If not, couldn’t we conclude it’s pretty spherical – at least from our perspective?
Spheres seem to be a pretty common thing in space: moons, planets, stars even galaxies seem to show signs of spherical origin (at least to me) – why leave the universe out? ;)
– rey
Tony,
Forgive me, I just realized something…
Thank you in advance for your response.
– rey
Now that we know the Universe is Flat, we can disregard Einstein’s “Curved Space” being responsible for gravitational lensing. Because, if the Mass of the Entire Universe is still not enough to curve space, what makes anyone think that the mass of a galaxy cluster would curve space?
I originally believed in a possible 4-dimensional sphere universe. If we somehow leaped off of the surface of the universe (3-dimensional) and off into the 4th dimension we could end up in the true center of the universe which is completely out of our visible reality analogous to the center of the earth being out of our visible land.
That’s just what I originally believed as a possibility.
If the universe was 4D-spherical, if we had a gigantic triangle made of laser somehow, we would be able to measure a sum of all 3 angles to be greater than 180 degrees—regardless of where we had it in the (visible 3D existence of the) universe.
i think this is hokey pokey. lmao. seriously, lasers proved its flat? we are seeing light from stars that are millions of light years away, how the F did we see light from a laser at the END of the universe.
but lets say its flat, in the sense that its shaped like a plate, or a table top vs a globe… then what if we are in a giant universe filing cabinet, and the next sheet over is an entirely different universe, what if we can just hop into it?
also every direction you look are stars from millions of light years away. every direction. so flat is a relative term, meaning the universe is shaped like a pizza box, 4 trillion light years wide and long, and 100 billion light years thick…
back to the light distance and what we can see, i havent found anything explaining how their laser show worked… (please email me any info)
ok so i looked into it and it clicked… they are talking about this theory…
the big bang happened, universe is a small nothing and then BAM its expanding, now in everyones minds we visioned it expanding like a BALLOON. and so they think shooting lasers out will let us see the “skin of the balloon” if thats its shape.
but we got back that its perfectly flat.
what i am getting is as follows.
lets say that skin on the balloon was 1 inch thick, well the inside of the skin would be closer together than the outside, so if you measured the 2 edges with a triangle made of lazers, you would have two different sized triangles, due to resistance. or because your measuring the skin you would have an odd shaped triangle.
once again this is huey, and the universe isnt flat, but rather perfectly expanding, and its just getting bigger. there is no “skin” but rather space, as weve all known it. so again still a huey thing.
sorry for the rants lmao.
Ok, like i said: –>IF<– we had a gigantic laser. I’m not saying that they really did use a laser.
As I read more into it, I’m starting to understand that it goes something like this:
1) Normal matter such as atoms, electrons, protons, neutrons etc. has “normal” Mass. Such “normal” mass warps space, so space around “normal” mass will “pull in” the “fabric of space” — likewise it pulls in matter (a pulling force of gravity), giving it positive curvature. In this case, if someone traces a perfect circle around the center of a body of high mass, then the circumference of such circle would be ever-so-slightly less than 2*pi*r. (This is positive curvature)
2) Dark Energy however, has a pushing force of gravity. So it has negative curvature. In this case, if someone traces a circle around a VERY LARGE area in a region containing a greater amount of Dark Energy than Normal Matter, then the over-all gravitational field covering such area would be repulsive, and the circumference of the circle would be ever-so-slightly greater than 2*pi*r.
With all that being said, it seems like the universe is rather wrinkly, full of many positive and negative curves… But the sum of all curvature of the universe averages to “flat”, like a wrinkly surface. My next question is: Did the universe star off (BIG BANG) as a little sphere? or did it start off like a flat mini-disk? – and continues to expand like a widening disk? If so, what’s at edge of the “disk”? Nothing? How fast is it expanding? as fast as or faster than the speed of light? It seems, the farther out something goes, the more “redshifted” everything becomes hence the faster it’s moving away — such that near the “edge” of the now-visible universe, the hot gasses or plasmas are moving so fast that the high energy radiation from such very hot gasses or plasmas is so severely redshifted that it is visible in the micro-wave spectrum rather than the x-ray or gamma-ray spectrum.
we are not out there, at the other end of the lasers. so from our point of view the lasers go strait. whats to say they didnt travel topsy turvey curvey, and as they went out and came back our eyes viewed them as strait…. what if it is flat. what if we send a ship out of it. as in take a pizza box. thats the shape of the universe. pull a slice out vertically…. then what happens?
if the universe is infinite is there really a flat universe possibility.
nothing i have found explains my questions.
did they send lasers up down, left right forward backward. etc… to find out where it ends and where it doesnt… nothing proved anything.
It seems to me like we are calling the universe flat because it is within our possibility of measurement and observation, just like man once called the earth flat.
What if, as with the earth hundreds of years ago, our point of observation on the sphere is so small that we can only measure it as flat?
What if the point that takes up all matter from the big bang were just a tiny point, taking up a trillionth of a trillionth of all space on the sphere? Could we possibly use triangles to measure something so large, with such a minute point of reference?
This is just my opinion, but I think the “fabric” of space is FLAT geometrically speaking, absolutely flat! However, If you’ve ever heard of the terms “Ether” or “Aether” or “Æther”, which would be the physical material of space – [Imagine that space is not some grid or any kind of flexible surface, but rather a Soup of Massless/Virtual particles.] – I believe that space “thins out”. And gravity, is merely an expression of that thickness. Things move towards a region of thicker “æther” which has less of a “stress” therefore things have less energy in such region.
Imagine this….. the harder you pull on a guitar string, the more you stretch it out – making it thinner; at the same time, sound travels faster on the string – therefore a higher pitch or frequency. E = hv (plank’s constant x frequency). So, higher frequency means higher energy. So, space (æther) is thinner (and tighter) farther away from mass; which yields a faster speed of light, and a faster rate of time (in natural oscillations), which is higher frequency and higher energy – which can by shown by the pound-rebka experiment to some extent.
But this concept as a whole, violates conventional science. O well…