A few questions about our theories

[AKRichard]

Hello all,

Please dont laugh if my questions make me look a little ignorant, but if I thought I had the answers I wouldnt be asking.

First off a simple (I hope) question: I have read that we believe that the expansion of the universe is accelerating, how do they come to this conclusion? The further from us the faster we see things receding from us, but the further out we go the further back in time we are seeing. Wouldnt this imply that things were moving faster in the past then they are now which would mean the expansion is decelrating?

Next: Is gravity a property of space or of matter? From what little I know of They would explain the force of gravity would be explained as the exchange of virtual particles (gravitons), now if that is the case, how would a black hole exert gravity? (since nothing can escape). I realize that by the same argument, even if gravity is a property of space, how would matter confer its presence to the space its in. Like I said, I dont have the answers just questions I would like to understand.

Third: If gravity is a property of space, wouldnt light loose energy simply by traveling through space? if energy=mass then it should generate gravity albeit very slowly, could this possibly bring the superluminal objects Ive heard about back to below light speed?

While on that subject is the redshift do to the doppler effect or by the expansion of the universe?

I have a whole slue of questions Id love to ask but Ill start with this and see what happens.

Thanks for listening.

Richard

[astromark]

:)...;)HA, HA, Ha, ha ha ha ha.:)(L)sorry and welcome...

You are not so ignorant, I would judge you well educated and brave. I can see your questions unfolding some dusty books...

Ist question; I have no idea. I read it. Have been told. It just is... Hmm...
2nd question; Gravity is a property of space. A distortion of the fabric of time and space. The black hole has distorted the space near it.
3rd question; No. Space is empty. Nothing stops the light. Unless it gets between you and the source, and then it blocks it all. Yes, Doppler effect. Next Please....Mark.

[Durakken]

Ok I'm gonna try to explain these...

Let's take a trip into imagination land and lets set up a scenario. In imagination land the universe is 100 light years at the first observation and all stars are set at the outer edge and you are at the center.

Now
if the universe was collapsing we would see the star rushing in and the light would be blue shifted
If it was stationary we would see the natural color of the light.
If the universe is expanding we would see the stars rushing away and the light would be red shifted.

Now here's a question, have the stars moved? They are at the edge still but they have moved away from us.

But that's besides the point your asking since we see galaxies closer to us moving slower away from us and those further away faster away from and since time is a factor how do we know that the universe is accelerating?

The easiest way to explain this i'd say is take two cars that sound exactly alike and place one at say 10 yards and one at say 20 and make both travel at the same speed and then different speeds. You'll notice the sound is different in most cases.

The problem you present is that it takes such a long time or light to reach earth, how do we know something hasn't changed. This is true We don't but according to our observations it continuously becomes more red shifted and to ask how do we know it hasn't changed is much like asking how do we know that the sun will rise tomorrow morning. We don't but we assume since it has and continues to do so it most likely will.

Second question:
Gravity is caused by matter residing in space-time. The more matter in one area the more gravity that is in that area. Hypothetically if we could remove matter from space-time and observe it without it we wouldn't see gravity and likewise if there was no matter in space-time there would be no gravity...so it's not really a property of either but rather an effect caused by a combination. Think of it sorta like flavors of foods... if you take one flavor and taste it and then another flavor and taste it... the combination of the two will create a new flavor that isn't quite a property of either but rather a fusion of both.

third...

Light bends space-time as space is expanding...And energy can't lose energy >.>

redshift is caused by the doppler effect which is caused by the expansion of the universe...

I think you know all this and i think there are better explanations but i think this is sufficient...

[Eroica]

I have read that we believe that the expansion of the universe is accelerating, how do they come to this conclusion? The further from us the faster we see things receding from us, but the further out we go the further back in time we are seeing. Wouldnt this imply that things were moving faster in the past then they are now which would mean the expansion is decelerating?
It is true that the more distant an object is the further back in time it emitted the light that we are now receiving.

But you must remember that that light contains a record of all the cosmic expansion that has taken place during its lengthy journey across space to us. Because it has experienced almost all of the cosmic expansion, it has become highly redshifted.

Light from a less distant object has only been travelling through space for a fraction of the Universe's lifetime. Therefore, it has only experienced the recent cosmic expansion, so its light is less redshifted.

Now, if the expansion were constant, the light from an object 2 billion light-years away ought to be twice as redshifted as light from an object 1 billion light-years away. During the first billion light-years the distant object's light will experience a billion-years' worth of cosmic expansion, while the nearer object has not yet emitted the light that we are now receiving. During the second billion-years, both objects' light will experience the same amount of redshift.

But observation shows that the light from distant objects is less redshifted than this. This implies that their light experienced less redshift during the period when it was travelling through space and the closer object had not yet emitted its light.

[NEOWatcher]

Please dont laugh if my questions make me look a little ignorant, but if I thought I had the answers I wouldnt be asking.
I won't laugh as long as you take my advice first...


Third: If gravity is a property of space, wouldnt light loose energy simply by traveling through space? if energy=mass then it should generate gravity albeit very slowly, could this possibly bring the superluminal objects Ive heard about back to below light speed?

That sounds a lot like "tired light". Use that as a google search on this site, and you'll find lot's of discussions that address it.

Even if that's not what you're getting at, it should have plenty of the answers you are looking for.

[Cougar]

First off a simple (I hope) question: I have read that we believe that the expansion of the universe is accelerating, how do they come to this conclusion?
I think Eroica answered this well. It's not that simple a question. It was only figured out 10 years ago. The finding seems to be holding up. Here's a little more explanation on that:

With light curve shape analysis and other techniques, supernova Ia are quite good standard candles. This means their distance can be determined by their apparent luminosity. Of course, their distance is also determined by their redshift according to Hubble's law. Their distance according to their apparent luminosity indicates that they are farther away than they should be according to the Hubble redshift-distance relationship (because they're dimmer than they would be if the Hubble constant was actually constant). Well, if they're further away than the Hubble relation says they should be, then the Hubble relation, i.e., the expansion rate, must be increasing.

You may have to read that twice. :o


Next: Is gravity a property of space or of matter? From what little I know of They would explain the force of gravity would be explained as the exchange of virtual particles (gravitons), now if that is the case, how would a black hole exert gravity? (since nothing can escape). I realize that by the same argument, even if gravity is a property of space, how would matter confer its presence to the space its in. Like I said, I dont have the answers just questions I would like to understand.

Remarkably, scientists don't know exactly how the mechanism of gravity works. With first Newton and then Einstein, we can predict extremely accurately the effect of gravity, but how it works, :confused: .

In the highly successful Standard Model of particle physics, three of the four known forces of Nature make their presence known via different force-carrying bosons, all of which have been detected. This seems to suggest that the fourth force (or interaction), gravity, should also have a boson, dubbed the graviton, but this has yet to be detected and is still hypothetical. If gravitons are the answer, they can obviously escape a black hole since black holes exert a tremendous amount of gravity.

Clearly the amount of gravity is determined by the amount of mass. If gravity is a property of space, somehow mass tells the surrounding space how to "curve."

[Tim Thompson]

... if my questions make me look a little ignorant ...
Ah, but if you didn't ask, then it would not be simply a matter of "looks"; you would actually be more ignorant!


I have read that we believe that the expansion of the universe is accelerating, how do they come to this conclusion?
Everything we think we know about cosmology comes from a simple process:

Observe the universe as precisely and accurately as technology allows.
Create a working model of the universe derived from known physics.
Predict what the observations would look like if the model were "true".
Compare the observations to the predictions.
If they compare favorably - Hurray!
If they compare poorly - Boo!

So, we can never say for certain what the universe really is or what it is really doing. All we can do for sure is to do the comparison, and then if the comparison is favorable, then we can pretend that the universe is really doing what the model says it is doing, and we won't make any big mistakes that way.

Now to the question. We make a model of the universe, based on known physics, and from the model we derive an expected relationship between distance (derived from the model) & redshift (observed). If we can then figure out a way to actually measure the distance (hard to do for cosmology) then we can compare the observed redshift-distance relationship to the model redshift-distance relationship. The difference between the model and the observations tells us that the expansion of the universe is accelerating, based on the reasonable assumption that the observations are "truth", not the model, which has to be adjusted to agree with the observations (the adjustment being the addition of recent acceleration). This conclusion was originally published in Riess, et al., 1998 (http://adsabs.harvard.edu/abs/1998AJ....116.1009R).

Measuring cosmological distances is pretty hard to do. But we have good reason to believe that type-Ia supernovae are all about the same intrinsic brightness. That makes them "standard candles", and knowing that we can derive the distance to the supernova from its observed brightness. So far that looks like a reasonable thing to do. However, if that reasoning ever breaks down, if we discover that type-Ia supernovae are not standard candles, then the acceleration of the expansion of the universe will vanish at once.


Is gravity a property of space or of matter?
Both, assuming the validity of general relativity. Mass (which is not necessarily "matter") causes a deformation of space, and we experience that deformation as the "force" of gravity. So in general relativity gravity is the result of the geometry of space, and not really a "force" as we normally think of forces. Of course it must be remembered that this does not mean the space is "really" deformed or bent or curved or anything else. It does mean that the working models of the universe based on the known physics of general relativity treat the universe as if space were deformed, and when we do that our predictions and observations compare favorably. Hurray!


If gravity is a property of space, wouldnt light loose energy simply by traveling through space? if energy=mass then it should generate gravity albeit very slowly, could this possibly bring the superluminal objects Ive heard about back to below light speed?
The answer to the first question is simply yes, and light does lose energy as it moves through space, because the space stretches as the light moves through it, stretching out the waves, and longer wavelengths have less energy. At least, that's the way the models work, and that's what we see when we observe. So far so good.

As for the rest, I have no idea what superluminal objects you have heard of so I have no way to know how to answer. If you can provide more information about that, we can give the answer a try.


While on that subject is the redshift do to the doppler effect or by the expansion of the universe?
Cosmological redshifts are caused by the expansion of space. They are not Doppler shifts.

[Ken G]

Please dont laugh if my questions make me look a little ignorant, but if I thought I had the answers I wouldnt be asking.That your questions are not ignorant is evidenced by the variety of different answers you are getting! In fact, questions like this are so profound that there actually is no "right" answer, but there are a range of answers depending on how deep you want to get into it, and how much is actually known at the moment.

The further from us the faster we see things receding from us, but the further out we go the further back in time we are seeing. Wouldnt this imply that things were moving faster in the past then they are now which would mean the expansion is decelrating?Your question exposes a misconception that when we see something far away, we are seeing a "snapshot" of the rate of expansion of the universe when the light was emitted. As Eroica and others explained, that is an unfortunate picture, and it essentially mistakes "speed" for "rate of expansion". The fact that the object has a higher speed is directly related to it being farther away, because if everything we saw had the same speed, they could not all come crashing together at the same time if we "run the clock backward". Since the universe evolves as a single thing, following the same behavior everywhere, it is necessary for more distant things to be separating from each other faster, like the ends versus the middle of a rubber band. So what you are seeing is just the "Big Bang" in action-- the acceleration is a small perturbation on top of what you are describing, in case that wasn't clear from the explanations above.


Next: Is gravity a property of space or of matter? All three are related, that's all you can really say. One might just as well ask if matter or space are properties of gravity (for example, Mach's principle tries to understand the existence of inertia, which is essentially what we mean by mass, as a result of the gravity of the distant cosmos, and the jury is still out on whether or not that makes any sense).


They would explain the force of gravity would be explained as the exchange of virtual particles (gravitons), now if that is the case, how would a black hole exert gravity? (since nothing can escape). Nothing real can escape. Virtual particles follow different rules.


Third: If gravity is a property of space, wouldnt light loose energy simply by traveling through space? The energy that light "possesses" is not actually a unique concept. Two different observers, in two different places or even in the same place, might conclude that light has a different energy. So there is nothing absolute you can say about the energy of light without also describing who is observing it. Gravity affects the answer you get, but not in a way that can be stated as simply as "light loses energy travelling through space".


if energy=mass then it should generate gravity albeit very slowly, could this possibly bring the superluminal objects Ive heard about back to below light speed?Generating gravity is not the same as losing energy, you may be thinking of gravitational waves, which are very different (real versus virtual gravitons, once again). And there is no need to bring the superluminal objects back below light speed, because they are only superluminal in regard to a particular way of coordinatizing space. I can choose a coordinate system where you and I are, right now, separating from each other superluminally, I'll just use a spatial coordinate that is being "sucked into" a superstrong imaginary vacuum cleaner located right between us. There's absolutely nothing "wrong" with that, I may coordinatize space any way I like.


While on that subject is the redshift do to the doppler effect or by the expansion of the universe?That's another toughy, and I don't think you have the right answer to this one yet. I would say that the closest we could come to a "right answer" to this one is that redshifts and expansion appear together, like mass and gravity-- two halves of the same coin. There is no point in trying to decide which causes which, indeed how either is "caused", in any kind of absolute way. We observe redshift, we observe expansion, and we have a theory that makes sense of the connection. But that theory can use totally different sounding words about "what causes what, and how it does it", and still be the same theory. That may be the most important lesson of all from relativity. For example, I can easily claim (as many do) that expanding space causes redshift, but I think I would more accurately claim that the redshift and the expanding space have no direct causal relationship as they are both caused by the same other thing: the rules of the universe mediated by gravity.

[AKRichard]

OK that answers most of it in a way I can understand except when you are talking about the redshift being from the exapnsion of the universe.

If I am understanding correctly, the expansion is like a "stretching" of the existing space instead of more space being created. and if it is "stretching" then wouldnt the ruler (for lack of a better word) that measures lights frequency also stretch, what is it that Im not getting on that point?

NEOWatcher I have read quite a bit about tired light and that was what led me to think of that question, that is not to say that I am an advocate to tired light, even I, with my limited education, can see some major issues with it, but, what I was thinking was if energy and mass are equivalent then wouldnt energy (light) emit gravitational waves and loose energy in the process?

Sorry I havent figured out how to do the quotes yet and next time I post a question I will include links to the articles I mentioned. Like I said I dont claim to know a lot about what I am talking, but I do have an insatiable thirst to figure things out, and I thank all for the replies.

Richard

[Ken G]

If I am understanding correctly, the expansion is like a "stretching" of the existing space instead of more space being created. There's no way to distinguish those possibilities. In fact, many people take the concept of "space" too literally anyway, space is not something we measure, it is something we conceptualize to understand the things we do measure-- and we have a certain latitude in how we choose to conceptualize space (connected with the idea of how we choose to give it coordinates, and what approach we choose to measure distances, as there are several). If someone tells you that existing space is being stretched, they are not telling you a fact about reality, they are merely giving you a useful and effective picture you can apply to help you understand cosmology. That's just how it is, those who want it to be more than that are simply making believe.


and if it is "stretching" then wouldnt the ruler (for lack of a better word) that measures lights frequency also stretch, what is it that Im not getting on that point?When you choose to use the stretching picture, the stretching has to be relative to something that isn't stretching, or it doesn't mean anything at all. The something that isn't stretching is rulers and bound systems of all sorts-- including solar systems and galaxies.

(To do quotes, just go to the post you want to quote and click on the "quote" button. You'll see how to proceed from what that gives you.)

[AKRichard]

When you choose to use the stretching picture, the stretching has to be relative to something that isn't stretching, or it doesn't mean anything at all. The something that isn't stretching is rulers and bound systems of all sorts-- including solar systems and galaxies.

(To do quotes, just go to the post you want to quote and click on the "quote" button. You'll see how to proceed from what that gives you.)

Thanks for the info about quotes, now back to the idea about stretching, so what you are saying is that as space expands the mass within it is NOT expanding in the sense that its (talking about the mass) volume increases? so does this expansion only affect space and electromagnetic radiation? If space and gravity are so intimately tied together does the expansion also affect gravity?

I warned y'all that I had alot of questionsso please forgive me for throwing more out there.

[Ken G]

so what you are saying is that as space expands the mass within it is NOT expanding in the sense that its (talking about the mass) volume increases? Right.

so does this expansion only affect space and electromagnetic radiation?It affects matter too, on scales that are unbound (basically pairs of well separated galaxy clusters). Before you wonder how the expanding space tells those distant galaxy clusters to separate, let me say that I think that would be putting the cart before the horse-- the reason we choose a model of space that is expanding (or really, a coordinatization of space that is expanding) is because of the separating galaxy clusters, not the other way around. It's all one big self-consistent picture, that's the key point-- there isn't a cause and effect there. Same thing for the redshift. If you want cause and effect, I think (I'm not an expert) the best you can do is say that the initial conditions of the universe are the cause, and the effect is how everything (matter, the "space" we imagine, light) evolves according to the equations of general relativity. Looking for finer shades of cause and effect between those components is not uniquely describable and probably not of much physical significance.
If space and gravity are so intimately tied together does the expansion also affect gravity?Sure, gravity has to be solved self-consistently with the expansion, it's all in Einstein's field equations (don't ask me how).

[Chris Hillman]

Ditto Cougar, Tim, and Ken, plus this:



From what little I know of They would explain the force of gravity would be explained as the exchange of virtual particles (gravitons), now if that is the case, how would a black hole exert gravity? (since nothing can escape).


By default, if no other gravitation theory is mentioned, everyone is likely to assume you are asking about gtr, which is a relativistic classical field theory. The notion of "graviton" belongs to the realm of quantum field theory. For the rest of your question, see How does the gravity get out of the black hole? (http://www.math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_gravity.html) from the Usenet Physics FAQ (http://www.math.ucr.edu/home/baez/physics/).



If gravity is a property of space, wouldnt light loose energy simply by traveling through space?


See Errors in Tired Light Cosmology (http://www.astro.ucla.edu/~wright/tiredlit.htm) from Ned Wright; you should bookmark at least three of his pages:

Cosmology Tutorial (http://www.astro.ucla.edu/~wright/cosmolog.htm)
Cosmology FAQ (http://www.astro.ucla.edu/~wright/cosmology_faq.html)
Errors in some popular attacks on the Big Bang (http://www.astro.ucla.edu/~wright/errors.html)




if energy=mass then it should generate gravity


Yes, the field energy in the EM field gravitates, if that is your question.



could this possibly bring the superluminal objects Ive heard about


You might be thinking of tachyons (a dubious theoretical speculation) or perhaps http://en.wikipedia.org/wiki/Relativistic_beaming



While on that subject is the redshift do to the doppler effect or by the expansion of the universe?


Answered in the UseNet Physics FAQ, and also in the Cosmology FAQ. You should probably study the resources I linked to above before you ask more questions.