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
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5 opinions for How Do We Know the Universe is Flat?
How To Weight the Entire Universe All At Once
Nov 19, 2007 at 1:27 am
[…] Measure the spatial curvature, the shape of the universe. This will allow you to get a handle on the density, the number of particles per unit volume of space. […]
rey
Dec 31, 2007 at 10:27 pm
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
rey
Jan 2, 2008 at 9:19 pm
Tony,
Forgive me, I just realized something…
Thank you in advance for your response.
- rey
Sonnie Pribadi
Jan 22, 2008 at 2:06 am
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?
Sonnie Pribadi
Jan 22, 2008 at 2:17 am
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.
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