Slashdot Mirror
Universe is Flat
D Anderson n'Swaart writes: "BBC News is reporting that a recent experiment called Project Boomerang, conducted with a super-sensitive telescope suspended 40 km above Antarctica, has provided powerful new evidence to support the current trend in scientific thinking that the fabric of space is essentially flat, and not curved as Einstein postulated. The one billion measurements gathered took three weeks to analyse on a Cray T3E supercomputer, and have provided insights on the creation of the universe, and suggest that it will continue to expand indefinitely without collapsing in a Big Crunch."
Rather, what it means is this: Einstein's theory predicts that spacetime is curved. However, some curved spacetimes can be sliced up into "space" and "time" in such a way that SPACE is flat. Current scientific thinking is that SPACE is flat in this way (or very close to it) -- spacetime is still curved and the curvature obey's Einstein's equation.
Of course, it is true that Einstein personally favored a closed universe model in which space is hyperspherical and hence curved, so in that respect the above statement is correct. But Einstein's theory admits many other solutions and experiments favor near-flatness.
GR is believed to be wrong. That's why we've been trying to discover the Higgs Scalar Boson, among other things, so we can figure out exactly how it is wrong and what should replace it.
Steven E. Ehrbar
The gravity of the universe is just enough to make the universe flat. If there was more mass, the universe would be curved one way (closed), and with less mass, it would be curved the other (open).
You are right, this is a universal flatness, not a local flatness.
If tits were wings it'd be flying around.
I got up this morning and discovered that my tire is flat, the coke I opened last night is already flat, my G/F is still flat,
I think I'll spend the day compiling kernels, just to take my mind off my troubles.
--
Sheesh, evil *and* a jerk. -- Jade
With out a Big Crunsh, that means that the end of the Universe will be a cold one - eventually, all the stars will die out and the universe will eventually become a uniform plasma of quarks and electrons(this is dictated by the laws of Thermodynamics - the Universe is going towards a state of increased entropy - that means low energy - and a quark/electron plasma is thought to be at the lowest possible energy).
I think the discovery can be put more precisely without too much confusion. I don't doubt that you understand all that is meant by the universe being "on overage" flat, but the experiment has broader implications than this post makes clear.
A flat universe is flat in the absense of something that curves it, while a curved universe is curved even in such absense. That is, mass is responsible for all (?) of the curvature (one effect of which is gravity) which we see, in a flat universe. In a curved universe, space is warped even in the absense of a mass (travelling objects appear to be deflected by the gravity of a mass which isn't there).
We've of course already heard this news about a year ago when the most powerful instruments we had at the time said that the curvature was as close to zero as we could measure. This experiment uses more powerful instruments, confirming that the curvature is even closer to zero than we could accurately say before.
This is actually one of the few experiments which says that the universe is not as wierd as the wierdest imaginable possibility. A flat universe is what most people expect: things go in straight lines unless something (in this case, gravity) interferes with them. If any of the wierdness that Einstein proposed (trajectories bent by nothing but space itself) actually occurs, it is too small for us to measure.
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under-paid karma whore
Yes, the article was misleading on this point.
A corrected version of the BBC sentence would be:
The experiment was about real light, not light in some hypothetical 'empty' space.
For astronomers, a flat universe also means that the negative curvature of geodesics being pulled apart by the expansion is exactly balanced by the positive curvature from gravitational attraction (and the expansion continues forever, but with no exponential runaway).
You might like to look at this press release from Berkeley:
- MAXIMA, a balloon-borne experiment directed by UC Berkeley, finds evidence for a flat universe, inflation and a cosmological constant.
Alternatively search google for "Boomerang Cosmological Constant flat".Such "dark energy" acts as a source for gravitational attraction and lensing, but it also exerts an outward pressure similar to the Caisimir effect, which accelerates the expansion.
You seem to be confusing two issues: (i) the 'average' flatness of the universe, and (ii) the cosmological constant.
The question of the flatness of the universe question is basically this: if you defined a triangle of geodesics, would the angles in the corners add up to more than 180 degrees (positive curvature: a 2d analogy would be the surface of a sphere); less than 180 degrees (negative curvature, cf the 2d surface of a saddle); or exactly 180 degress (like a 2d sheet of paper).
It turns out, when you do the sums, that an 'empty' expanding universe would not be flat, but would have negative curvature: two geodesics emerging from a point are pulled away from each other. For an expanding universe to be flat, there has to be enough gravitation attraction to balance this pulling apart. If the average energy density is a little higher, the attraction becomes more important than the divergence, leading to positive curvature and a closed universe.
These latest results confirm that the universe appears to be either exactly or very nearly flat. This implies that the average energy density is either exactly or very nearly at the critical value. But the amount of visible matter seems to be too small to account for this. So it has been suggested that there is either more mass in the universe than we can see - "dark matter" - or that empty space itself has an energy density (like a quantum particle's zero-point energy). The present results, by indicating that space is flat, tend to support this idea of a missing energy density. This leads to an extra term in the equations, the Cosmological Constant (Lambda), which appears in addition to the energy density terms arising from visible matter and radiation. A flat universe suggests that Lambda is not zero.
"on average" flat
Strictly speaking, the discussion above assumes a Friedmann model of the universe. This is a simplified model which assumes that the universe is and remains homogeneous -- the same everywhere, with the same density at every point. Normally in relativity there is no way to say whether events in different places are happening at the same time; but in the Friedmann universe one can use the local density of matter (which is the same everywhere) to define a universal "proper time" co-ordinate. Having nailed down the co-ordinate system, one can then separate the spatial and the temporal aspects of the equations.
Of course, the universe is not homogeneous when you look at it closely -- every so often you come across a planet or a star or a black hole; and getting too close to the latter can bend geodesics a lot (gravitational lensing). Nobody is suggesting anything but that the universe is curved "up-close". But on a larger length scale the Friedmann approximation seems to work well: very few geodesics will actually pass close by a black hole; most will indeed experience an "average" amount of bending.
There also is the issue of the shape of the universe, which is what this story addresses. The balloon analogy describes a closed and unbounded universe. There are also other types such as the closed and bounded (such as an expanding dinner plate) and an infinite universe.
This all leads to some interesting questions such as what is this all expanding into? Is this 4-D space hung in a higher dimensional space, or is this all that there is? Some cosmologists look at possibilities such as whether our universe is just a local bubble and that there are other universes that are right next to us (and perhaps connected via black holes/quasars). Also, note that if the universe is as it is described above, then you should be able to set off in one direction and travel all the way around the universe and get back to your starting position, much like sailing around the Earth. However, with the rate of expansion of the universe, you would have to travel awfully fast. If the universe were not expanding and was closed (as with the balloon analogy) then if it were optically transparent and you had a big enough telescope, then you would be able to look far out and eventually see yourself.
There are a number of good layman-targeted books on this subject. Two of the most popular have been Hawking's A Brief History of Time, and more recently Brian Greene's The Elegant Universe. A very good book on the Big Bang itself is Steven Weinberg's The First Three Minutes.
The perception of time is an interesting topic because the "arrow of time" comes from thermodynamics (the second law in particular), and so some believe that it is independent of the expansion of the univese. However, some have entertained the possibility that if the universe were to contract, then time would reverse itself.
In addition to the books I referenced previously, if you really want to exercise your brain, I highly recommend Barrow and Tipler's The Anthropic Cosmological Principle. You might not agree with all that is in it, but it has some very nice historical chapters and discussions on the nature and fate of the universe.
>One thing that puzzled me with this analogy is
>that, for this to work, the galaxies had to be
>taped-on solid items. If galaxies are just
>collections of space (a bunch of dots) then why
>wouldn't the dots inside the galaxy fly apart, too
They are, but the force of gravity keeps them stuck together. In fact the planets are orbiting in stable orbits just slightly smaller (for their orbital velocity) than they would be if the universe wasn't expanding- the extra gravitation acceleration from being closer together cancels the expansion 'acceleration'.
So the upshot is that things that aren't in orbit drift apart. Things that are in orbit stay stuck that way at the same distance.
-WolfWithoutAClause
"Gravity is only a theory, not a fact!"Wait, sorry -- I thought it said the universe was PHAT.
Honorary Member of Jackie Chan's Kung Fu Process Servers
So the universe is flat, is it level?
Um, could someone draw me a picture of what they're talking about? :-) Don't worry, take your time...
And, are we talking more of a canvas, or light gingham?
Unfortunately, we can not see the big bang, as the young universe was not transparent.
The microwave background originates from the time when the universe is about 100 000 years old. In those early days, the universe was filled with hot gas and radiation. The gas was hot enough to be ionized, so it was very opaque. As the gas was getting gradually cooler, the electrons got bound with nuclei, forming neutral atoms and allowing the radiation to pass through. Now, this radiation has been doppler-shifted to the 3K blackbody we see. So, we can NOT see beyond the microwave background.
or I have to be able to see it in EVERY direction I look
At the moment of big bang the universe was point-like, or a singularity (Now I'm assuming this is the exact moment, ignoring all the stuff about Planck time etc.) I'm no expert in general relativity, but I think the concept of 'direction' can not be applied to singularities. The point of time at which the big bang happened is a point of time when the three spatial dimensions of the universe did not exist. No space, no directions.
I hope this clarified things a little. However, I get the feeling I'm trying to explain a zen koan to someone.
The 'black body' refers to the spectral distribution of the light energy (Planck's law), not colour of the radiation. For example, the Sun is a rough blackbody at 6000 Kelvin. The microwave background is a blackbody that was originally at a temperature of the order 10000 K, now redshifted to 2.73 K.
how far are we looking back then?
The exact distance depends heavily on the cosmological model used, and there is no consensus on which model to use. So the values I give may be different from what you see by an order of magnitude or even more. However, the universe was about 300.000 years old at that time, and the distance is a few tens of (American) billions of light-years.
This seems to contrast the age of the Universe, (how can we see 20 billion light-years away when the light has travelled only 10 billion years?) but this is were the cosmological model (Hubble parameter, spacetime curvature etc.) steps in.
> If you constrain yourself to the surface of the
> balloon...then as you inflate the balloon you'll
> see that this universe expands and all the
> galaxies fly apart from each other.
One thing that puzzled me with this analogy is that, for this to work, the galaxies had to be taped-on solid items. If galaxies are just collections of space (a bunch of dots) then why wouldn't the dots inside the galaxy fly apart, too (and thus, to your viewpoint, the universe would stay constant w.r.t. the size of the galax.
And if galaxy, why not your body?
I do recall somewhere that galaxies rotated as if a solid disk, rather than as if a whirlpool, which would be the case if they were in orbits around a cenrtal area.
I am for the complete Trantorization of Earth.
Ryan T. Sammartino
Ryan T. Sammartino
"Ancora imparo"
Other information on this project can be found here, here (Caltech), or here. This link to Princeton University seems to explain the project much better, at least to me.