But aren’t we the dead center of the observable universe? Isn’t the observable universe isotropic?
Well, sure. But we’d be in the center of the observable universe if we were someplace else, too. It’s kind of like Paris Hilton and the paparazzi; the only reason she’s always on the front page of People magazine is because Paris only exists to be on the cover of People.
Stranger
And if someone were on a planet near the edge of our observable universe their observable universe would be isotropic out to 14 billion light years so there must be parts the universe beyond our ability to observe?
The most distant objects we can observe are currently somewhere around 45 Bly distant (in current time), though at the time they emitted the light we now observe they were much closer. (The Wikipedia article on the observable universe says 46.5 Bly and 40 Mly respectively, but I don’t know where they get there numbers from.) The cosmic event horizon isn’t a fixed boundary in space but a moving front, and if the inflation of space is in fact accelerating the boundary will be moving progressively toward us. Note that this boundary is a barrier, and just like the event horizon of a black hole is has "no hair’; that is, we can’t ever get any information from beyond it, or at least not without some super hinky faster-than-light propulsion that would equate to open-ended time travel.
I’ve read wildly varying estimates of the overall size of the universe (which is based upon total mass and an assumption of more-or-less uniform distribution) but I don’t think that there is any real confidence in those numbers. It’s not as if the universe is a circle whose diameter can be measured, anyway; the expansion is occuring in four dimensions such that nowhere in the universe is there an “edge”; it’s all just non-overlapping local regions that are inexorably expanding to a horizon.
Stranger
The trouble is, we apparently don’t know a whole lot when it comes to interstellar energy. Things like dark energy are totally unknown. So there needs to be independent varification of distances.
All current observations are still consistent with an infinite Universe. So any number anyone might estimate for the size should have infinite error bars (at least above). Some results do favor a smaller Universe, but they’re not definitive.
So we know that the universe is expanding because of the redshift, and we know that nearly everything is redshifted because the universe is expanding… How isn’t this circular logic?
Measured Cepheid distances go out to at least 38 million light years. Type 1a Supernova distance measurements work out to at least 11.3 billion light years. Correlations between different methods of measuring cosmic distances are used to escape the circular logic trap.
Redshift makes for a handy yardstick because it doesn’t depend on having a supernova, variable star, or whatever in the object you’re looking at, it works on about anything you can see.
You’re using two different senses of “because”. The way we know that the Universe is expanding is that almost everything is redshifted. The way we know that almost everything is redshifted is by looking at them.
The cause for us observing redshifts is that the redshifts exist. The cause for the redshifts to exist is the expansion of the Universe.
That would certainly be a big help. However, the idea of an expanding universe fits all of the data as far as I know. There could be a hitch that is being glossed over with arm waving I haven’t heard of.
And yes, there is a lot more that we don’t know that that we do. However, so far it appears that the expanding universe allows predictions as to the properties of newly discovered objects and is fruitful of further investigation. More of a scientific theory can’t be asked.
The last eighty years of astronomy may be of interest to you.
I don’t think it is appropriate to dismiss human_extinctions comments so surreptitiously. We aren’t all astrophysicists. I will trust the astrophysicists to a certain extent, but as far as I can tell, they know less about the intergalactic universe than I know about subatomic particles. How a photon of light behaves across thousand light-years of intergalactic vacuum is just not something we can set up in the laboratory.
Just a minute. Science isn’t founded on logic. The fact is that very careful measurements were made of the red shift of objects the distance of which was directily measureable. Measurements were also made of redshifts of objects that contain variable stars whose absolute luminosity is known because of their period-luminosity relationship. This relationship was also based on measurements of the period-luminosity relation of variables whose distance could be directly measured. The redshift-distance curve was based on experimental data. So there is a lot of confidence that the redshift-distance connection is real. Now, once you have verified by every means available to you that the redshift curve is valid, that curve can be used to estimate the distance to galaxies that are far enough to be beyond our ability to measure distance directly or to resolve variable stars.
As to possible changes in the characteristics of the light over time, the spectra of distant objects are identical with the spectra of near ones. For example if we take the spectrum of a star in our galaxy that’s only maybe 100 light years away we might get lines at wavelengths 1, 1.3 and 2 having the intensities respectively of 3, .5, and 1. These are made up numbers for illustration only.
Now we can measure the spectrum of a star 1 million light years away and we find spectrum lines at wavelengths 1.3, 1.43, and 3.9 having the intensities 1.5, .25, and .5. These wavelengths are the wavelengths of the star at 100 ly multiplied by 1.3, i.e. redshifted and the intensites all multiplied by .5. If there were absorbtion of some of the energy from light it’s asking quite a bit that all wavelengths would undergo the same absorbtion. It’s not hard to draw the inference that the physics of the light emitted 1 million years ago is that same as the physics of the light emitted 100 years ago. I suspect that the values of the intensities of the spectrum lines might not scale exactly 0.5 for all lines but as far as I know there isn’t a difference not resulting from unavoidable instrument errors, measurement noise and the like.
The same thing holds for the spectra of galaxies whose light was emitted several billion years ago. So yes, it’s not impossible that the physics billions of years ago was different. In fact if you go back far enough and get closer to the Big Bang I think it was. However all of our experience says that the physics on which the redshift-distance relationship is based hasn’t changed with time.
By the way, I get the impression that you think scientists are 100% positive that all of this is unquestionably true. I don’t think that’s the case at all. People are poking away at this continually because it’s the nature of some scientists to question the status quo.
If that is directed at me: not at all. I question as much as I can because it is obvious that we still know little, and we are wrong about a lot. Not nearly enough physics (and therefore, cosmology) is verified by scientific experiments (real science, not math experiments like string theory, which is laughable).
I’ll go over the comments that I missed when I have a bit more time. Thanks for your replies (and I’m glad there is a forum that can acually hold a debate at a certain level of intellectuality).
You are talking about cephid stars, and they are not good for long distances. No matter Chronos said they use supernovae to do essentially the same thing.
Except for the red shift and lower intensity. I am not an astrophysicist, and will generally trust the accepted theory. I also know that that theory could change.
No, I wasn’t referring to you. However, the development of the use of redshift to infer distance wasn’t a logical exercise, it was ultimately based on experience so it isn’t a case of circular reasoning. It is, however, extrapolation from what is solidly known to what is much less well known and, as I said, scientists realize this and there are those who looking into it. Not many, I would guess, because it’s not the way to making a name unless to happen to make an astonishing discovery.
The mathematical “experimentation” in cosmology is a subject of continued debate among abstract theorists, experimentalists and those embracing both. It seems to me the best course is the conventional one that has been followed for a long time. Make guesses as to why things happen. Make predictions based on those guesses. Check to see if the predictions are confirmed. Write it up, get it published and let everybody have at it.
I read the question as blue and red states, so was quite confused there for an entire page. Still not sure I’m comprehending all the astro-physics, but I know this won’t keep me up at night.
Query: if we can only see things in relation to us, then we are indeed the center of things, right? (not the actual center, but our perspective is from there, no?)
Every point in the Universe has an equally strong claim to being at the center, since every observer observes emself to be at the center of es observable Universe.