Starship Asterisk*

or it may just fall onto some alien planet and, if not destroyed in the atmosphere, considered by aliens to be a message from alien gods and used as a corner rock to build the main temple of their alien religion.

Chris wrote:Well, the observational evidence is strongly in favor of a flat universe (that is, flat in four dimensions), so "oval" doesn't really seem to work (assuming the shape is extended to its 4D equivalent; certainly there is no reason to think that in 3D the Universe has any shape at all).

If the universe is flat, it could be a triangle, or a wreckedtangle.. in which case we could be near one corner. Perhaps we will soon see the outer limits of our corner, while the other corners will lie undiscovered.

Chris wrote:Not the Universe, but the observable universe. The two are probably very different."

Your "Probable", Chris, is an extremely wild guess at most, far more speculative than my anti-gravity bubbles which can account for what we do see, even if I can't do the math to support the idea, but which I have described in detail.

Chris wrote:'... Speculation' is the wrong word for both DE and DM, however. These are solid concepts backed by observation and testable theory. They are far from speculative (as most people would interpret that word), even if there remains much research to understand them (or the effects attributed to them)."

Speculation is the perfect word for Dark Matter and Dark Energy .. they are as much speculation as my anti-gravity bubbles .. they are ideas which seem to fit the observations .. nothing more than that. IMOPO my anti-gravity bubbles have far more substance than DM DE, but perception is in the eye of the beholder. I won't attempt to persuade you of my perception, Chris, but you will not be able to persuade me of yours. But it is enjoyable conversing with you.

makc wrote:

aristarchusinexile wrote:Perhaps, if we do, we will find the universe shaped like an oval with us located in one end of the oval.

Nah, I think it is dodecahedron.

How about a Dodocahedrun .. a bird running towards extinction.

iknownothing wrote:I have been having a big argument with a friend about what is beyond the universe. I say nothing at all. He says the universe must be expanding into something and that that something is black. I think black is 'something', and so can't be beyond the universe. Help!

The Universe doesn't need to be expanding into anything. In the context of most current theory, the idea of "outside the Universe" is meaningless. There is speculation about the possibility of some sort of hyperuniverse, but that remains largely outside the bounds of what can be observed or tested- and may very likely always remain so.

I've said it before, and will no doubt say it again: the Universe is under no obligation to follow rules that seem intuitive to humans. Just because we can't visualize something without an "outside" doesn't mean such a thing can't exist.

makc wrote:or it may just fall onto some alien planet and, if not destroyed in the atmosphere, considered by aliens to be a message from alien gods and used as a corner rock to build the main temple of their alien religion.

I heard in the 1960s that diaphragms sent without instructions to India for population control were mostly placed on the altars among the fertility gods.

iknownothing wrote:a friend ... says the universe must be expanding into something

Tell your friend to blow up a balloon half way, take a magic marker and draw some galaxies all over it, and then continue to blow it up more. The surface of the balloon expands and the galaxies get farther apart. The surface of the balloon is expanding but it doesn't have an edge that is expanding into anything. Of course the surface of the balloon is only a two-dimensional metaphor for the three space dimensions of the universe.

makc wrote:I think it is dodecahedron.

According to the ancients:

Cube: Earth
Tetrahedron: Fire
Icosahedron: Water
Octahedron: Air
Dodecahedron: Universe

aristarchusinexile wrote:

makc wrote:

aristarchusinexile wrote:Perhaps, if we do, we will find the universe shaped like an oval with us located in one end of the oval.

Nah, I think it is dodecahedron.

How about a Dodocahedrun .. a bird running towards extinction.

Nah, its the seventeenth stellation of the icosidodecahedron.

My backround is philosophy, not physics. I started this topic out of a dissatisfaction with the alternatives of a "big rip" or a "big chill". This is a philosophic dissatisfaction. I would be ok with the big crunch, but I understand that at the moment the observations of acceleration don't seem to support this. The reason for my dissatisfaction stems from the idea of the big bang "beginning" the universe and one of these alternatives being its end. What would be the unmoved mover that caused the tiny spec to explode? Why then and not sooner? The reason I like the big crunch is that it opens the idea that there is a cycle. The universe expands, then contracts, crunches down until it cannot do so anymore and then explodes again. Thus the universe is eternal. It has occurred to me today that it could be eternal if at some point in the future, after the big rip for example, another big bang occured somewhere in the universe. If we have no idea how or why the first one occured there is no reason to suppose that it could not happen again.

I asked my brother-in-law ( physics Phd student at Harvard) the initial question regarding the idea of a universe surrounded by matter instead of a void. I got the same answer chris provided. Almost to the letter. He used the "isotropic" nature of our observations as well as Gauss's law of gravity not acting in a sphere. Thanks Chris.

Why is the universe thought to be expanding at an increasing rate (laymen's terms, please)? I understand that the greater the distance, the greater the red shift and relative velocity. But the greater the distance, the further back in time. So how does this equate to an accelerating expansion?

bystander wrote:Why is the universe thought to be expanding at an increasing rate (laymen's terms, please)? I understand that the greater the distance, the greater the red shift and relative velocity. But the greater the distance, the further back in time. So how does this equate to an accelerating expansion? :?

We can basically ignore time. What is of interest is the relationship between distance and redshift. Hubble's Law by itself suggests a linear relationship, v = H0 * D. But it seems that H0 has changed over time, which is another way of saying that the rate of expansion has changed over time. For a long time, people thought that the rate of expansion was decreasing, as you'd expect if the self gravity of the Universe was acting on everything. It's like what happens if you launch a probe from the Earth. Once it is no longer under power, it will slow down- even if it had reached escape velocity.

Then, about ten years ago, some people were looking closely at the redshift of distant Type Ia supernovas. These have (it is believed) a very uniform brightness profile, which means that if you know their intensity, you can calculate their distance. These supernovas were dimmer than expected when their distance was calculated using redshift and Hubble's Law. That meant they were actually farther away than expected, which means that the rate of expansion has increased with time. Since then, many measurements have been made, and it is pretty well established that the universal deceleration parameter is negative, meaning that the rate of expansion of the Universe is increasing with time.

Chris,
I read that when the universe began, shortly after the big bang, space was rapidly expanding. So rapidly in fact that it has been theorized to have been expanding faster than light.
I've also heard that we can only see so far back in time/distance due in part to this fact?
Presuming that the speed of expansion has decreased over time, wouldn't it be logical to presume that when we look 13.5GLY away, not only do we see the galaxies as they appeared 13.5 GLY ago, but wouldn't we also see the light reacting to the physics of the time? Due to the increased expansion rate in the early universe, wouldn't we expect to see those early distant galaxies moving away at a faster pace? Could this be utilized as an arguement against an increasing expansion rate now?

BMAONE23 wrote:Chris,
I read that when the universe began, shortly after the big bang, space was rapidly expanding. So rapidly in fact that it has been theorized to have been expanding faster than light.
I've also heard that we can only see so far back in time/distance due in part to this fact?
Presuming that the speed of expansion has decreased over time, wouldn't it be logical to presume that when we look 13.5GLY away, not only do we see the galaxies as they appeared 13.5 GLY ago, but wouldn't we also see the light reacting to the physics of the time? Due to the increased expansion rate in the early universe, wouldn't we expect to see those early distant galaxies moving away at a faster pace? Could this be utilized as an arguement against an increasing expansion rate now?

The inflationary period that is required by most modern BB theories is an important element in explaining why we can't see the entire Universe, although it isn't required to explain that.

We can't look to the redshift of early galaxies to learn anything about inflation, because it happened - and finished - in the first fraction of a second after the BB. After that, the expansion of space was simply following Hubble's law - albeit with a slightly different value of H0 than we see today. It was another several hundred thousand years before the Universe became transparent to EM, which marks the oldest era we could possible observe (at least, with EM). And the first galaxies didn't form for a few hundred million years. So the short answer is: by the time there was anything that we can measure today, the Universe was already behaving as we see it today.

Igotlostintheshuffle wrote:
How about a Dodocahedrun .. a bird running towards extinction.

stilllost wrote:Nah, its the seventeenth stellation of the icosidodecahedron.

Does it have big teeth?

Zarathustra wrote:I asked my brother-in-law ( physics Phd student at Harvard) the initial question regarding the idea of a universe surrounded by matter instead of a void. I got the same answer chris provided. Almost to the letter. He used the "isotropic" nature of our observations as well as Gauss's law of gravity not acting in a sphere. Thanks Chris.

Ask your brother if he's ever heard of Pascual Jordan. I'm interested in his response.

apodman wrote:

iknownothing wrote:a friend ... says the universe must be expanding into something

Tell your friend to blow up a balloon half way, take a magic marker and draw some galaxies all over it, and then continue to blow it up more. The surface of the balloon expands and the galaxies get farther apart. The surface of the balloon is expanding but it doesn't have an edge that is expanding into anything. Of course the surface of the balloon is only a two-dimensional metaphor for the three space dimensions of the universe.

But the galaxies don't seem to be expanding.

apodman wrote:

makc wrote:I think it is dodecahedron.

According to the ancients:

Cube: Earth
Tetrahedron: Fire
Icosahedron: Water
Octahedron: Air
Dodecahedron: Universe

http://j3000.com/x/apod/5_solids.jpg

I must confess, I was just stabbing in the dark, or maybe that was something I heard before, circling around my underconsciousness...

wiki wrote:Plato wrote about them in the dialogue Timaeus c.360 B.C. in which he associated each of the four classical elements (earth, air, water, and fire) with a regular solid. Earth was associated with the cube, air with the octahedron, water with the icosahedron, and fire with the tetrahedron... The fifth Platonic solid, the dodecahedron, Plato obscurely remarks, "...the god used for arranging the constellations on the whole heaven". Aristotle added a fifth element, aithêr (aether in Latin, "ether" in English) and postulated that the heavens were made of this element, but he had no interest in matching it with Plato's fifth solid.

Zarathustra wrote:The reason I like the big crunch is that it opens the idea that there is a cycle.

...which perfectly aligns to Nietzsche philosophy, isn't it, my friend Zarathustra but man, Nietzsche did not even believe in atoms - it is time for you to re-evaluate Nietzsche, there's much more knowledge available to us today than to him in XIX century.

aristarchusinexile wrote:

apodman wrote:Tell your friend to blow up a balloon half way, take a magic marker and draw some galaxies all over it, and then continue to blow it up more. The surface of the balloon expands and the galaxies get farther apart. The surface of the balloon is expanding but it doesn't have an edge that is expanding into anything. Of course the surface of the balloon is only a two-dimensional metaphor for the three space dimensions of the universe.

But the galaxies don't seem to be expanding.

The analogy is useful, but should not be taken too literally. A better example would be a balloon with points on its surface. As the balloon expands, the distance between the points increases. The points themselves, of course, remain points.

In the real universe, gravity is dominant over expansion in regions as small as galaxies. So galaxies don't expand with the space around them. Theory and observation are in agreement here.

Chris Peterson wrote:We can basically ignore time. What is of interest is the relationship between distance and redshift. Hubble's Law by itself suggests a linear relationship, v = H0 * D. But it seems that H0 has changed over time, which is another way of saying that the rate of expansion has changed over time. For a long time, people thought that the rate of expansion was decreasing, as you'd expect if the self gravity of the Universe was acting on everything. It's like what happens if you launch a probe from the Earth. Once it is no longer under power, it will slow down- even if it had reached escape velocity.

Then, about ten years ago, some people were looking closely at the redshift of distant Type Ia supernovas. These have (it is believed) a very uniform brightness profile, which means that if you know their intensity, you can calculate their distance. These supernovas were dimmer than expected when their distance was calculated using redshift and Hubble's Law. That meant they were actually farther away than expected, which means that the rate of expansion has increased with time. Since then, many measurements have been made, and it is pretty well established that the universal deceleration parameter is negative, meaning that the rate of expansion of the Universe is increasing with time.

Let me see if I've got this correct. Type Ia SNs are a type of "standard candle". The apparent brightness of observed SNs meant that the SNs were farther away than the red shift indicated. But all that means is that at some time H0 had a different value. I still don't see why it has to mean that the expansion of the universe is accelerating.

bystander wrote:Let me see if I've got this correct. Type Ia SNs are a type of "standard candle". The apparent brightness of observed SNs meant that the SNs were farther away than the red shift indicated.

Exactly.

But all that means is that at some time H0 had a different value. I still don't see why it has to mean that the expansion of the universe is accelerating.

If H0 is changing with time, that is just another way of saying that the expansion rate of the Universe is changing with time. After all, H0 is just the "constant" of proportionality between distance and redshift. There is now a body of data relating different distances to different redshifts, and that data shows an increasing rate of expansion- that is, there is a deviation from a linear Hubble's Law.

Chris Peterson wrote:If H0 is changing with time, that is just another way of saying that the expansion rate of the Universe is changing with time. After all, H0 is just the "constant" of proportionality between distance and redshift. There is now a body of data relating different distances to different redshifts, and that data shows an increasing rate of expansion- that is, there is a deviation from a linear Hubble's Law.

I'm still not clear. Does the data show that H0 is greater for closer SNs? or for farther SNs? I would guess farther. If H0 is decreasing with time, that would seem to imply an increasing rate of expansion (distance is greater for a given redshift).

Chris Peterson wrote:A better example would be a balloon with points on its surface.

As you know, I have a fine-point marker that draws very tiny galaxies. I could also extend the metaphor by arguing that the ink in my markers dries hard and solid, preventing the small marked areas on the balloon from expanding. But that would be more detail than I wanted to write.

Hi everyone!

What would we see when we look at the center of our universe (where the big bang happend) today?

This is discussed a lot, but it is not easy for everyone to visualize, so keep asking.

Back to the balloon metaphor I just used in another thread, the universe is like the surface of an expanding balloon. The big bang can't be located anywhere on the surface of the balloon. But it was at every point on the surface of the balloon when the balloon's size was zero and the balloon was only one point. Now the only direction to look for the big bang is not in any particular space direction but back in time.

I'm sure someone will provide more detail or a better explanation.