Science a GoGo's Home Page Forums General Discussion General Science Discussion Forum Are we in a cosmic bubble?

Then that puts us at a special time. Scientists generally try to avoid assuming anything special about our place in the cosmos.

Thinking about the time required for the bigverse to form its stars and galaxies. It seems to me that because of the high density of the bigverse its star formation would be much accelerated with respect to our universe. After all the density would be so high that it wouldn't take as long for enough gas to be accumulated to start star formation since the gas wouldn't have as far to travel.

Bill Gill

If the bigverse had progressed to the point where it was composed mainly of supermassive black holes, would this not exert more gravitational attraction than a universe composed mainly of galaxies?

No it would still have the same mass. When a star collapses to a black hole the only things that certainly conserved are mass and electric charge. I'm not sure whether you can say that entropy is conserved, but the entropy of the black hole is proportional to the area of the horizon. I believe there is still some question as to whether information is lost. Oh, yes I'm not sure about spin, but I expect angular momentum is conserved. But any way creating a black hole doesn't add mass.

Bill Gill

Sorry 'bout that. Not exactly an example of quality writing on my part...


No. Net gravitational pull is towards the centre of mass. If you had a perfectly uniform field of matter there would be no net gravitational pull, as gravity would be equal in all directions. The presence of a low-density region disrupts that, but if you do the math of a low-density region in an otherwise uniform field of mass, the centre of mass of the whole will be centred on the centre of mass of the low density region. It is the distribution of mass, more than the absolute amounts, which lead to net gravitational movements.


But it still would be. If you take into account the size of our observable universe (~12YL in radius), the region where we see what we see would be less than 1% of the total volume. Ergo, at best, the "special position" will always be about a 1% probability, regardless of the number of universes.

Bryan

How do you locate the centre of mass in a possibly infinite universe?

This was not a comment on your post, it was an admission to past circuitous thinking on my part. Not uncommon.

Gravity is though to travel at c. So the centre of mass of any region of space can be determined by the mass in the sphere around it equal to the radius "of the age" of the universe (in light-years). Mass farther away than that would not interact at all with the area you are measuring the COM for, as gravity from those regions would not have yet reached your COM.

I.E. the centre of mass of any region in our ~14BYO universe would be determined by measuring the mass distribution in a sphere 14BYL in radius, surrounding that region.

Bryan

This looks quite straightforward if you look for the centre of mass for an area of which the centre of the bubble is the designated point. Attraction would then be towards the centre of the bubble. What happens, though, if the designated point is close to the boundary of the bubble?

The same; assuming that the "surrounding" universe has relatively even density. In this situation the low-density region is surrounded by a 'C'-shaped region of higher density. The tips of the 'C' will experience a net gravitational pull towards each other, causing them to collapse inwards.

I was thinking more about this, and the more I think about it, the more I'm sure this model does not work. We can measure red-shifts of the nearest galaxies to us, and have a rough idea of how quickly that shift is accelerating. Based on those numbers alone, we would have to be in a region where the outer "dense" universe would be observable, if this model was correct.

To be more clear about that, if this model were correct, the accelerative effect of the outside universe would spread through ours at c, because gravity travels at c. The amount of time it would take the known acceleration of our universes expansion to accelerate the local galaxies to their current speed (generally around -300km/s) is far longer than it would take for light (and thus gravity) to travel the interceding distance to our own galaxy. Thus, this outer universe would *have* to be visible to us, if it existed, since both its gravity and photons would have had to arrive here long ago, inorder for the surrounding galaxies to be accelerated to their current recessional speeds. Since we observe these red-shifts, but not an outer galaxy, it likely does not exist.

I'd also point out that the pattern of red-shifts that we see don't fit the model. Gravity strength changes due to the inverse-square of the distance of separation - i.e. move 2X as close to the source of gravity, gravity increases 4X. If this model were correct, we should thus see a such a relationship in regards to galactic distance verses acceleration. For example, if galaxy 'A' was 2X closer to the outer universe as galaxy 'B', it should be accelerating 4X as much. We don't see that - acceleration appears to be linear for a given length of space.

Bryan

This suggests that the "designated point" close to the boundary could not exist, because the "C" would already have collapsed.

However, the "C" would not have collapsed because, in the bigger picture, it is part of a complete circle; so what would cause collapse in that area when a measurement was attempted?

You're half-right. The collapse would occur throughout the circle, thus causing the whole thing to collapse. That is why we would expect a denser "outer" universe to collapse - you'd have an "infinite" number of 'C's (due to travel speed of gravity), resulting on a net inward acceleration throughout the "interface".

Bryan

Well, that seems to squash the whole idea. It will be interesting to see what the proponents have to say.

The OP's article was from 2008, and nothing on the idea seems to have been published since...things that make you go "hummm"

Bryan

New info on the subject. Hubble data says we aren't in a bubble.
From Physorg.com

Using data from the Hubble Space Telescope researchers have refined the expansion rate of the universe, and found that it is too great to be accounted for by the bubble hypothesis.

Bill Gill

Thanks for the link, Bill. It makes interesting reading.

Bryan, how does the following quote fit with your idea of the bubble collapsing?

".....the cosmic bubble theory. In this theory, the lower-density bubble would expand faster than the more massive universe around it."

Cool. That's news to me. My theory predicts this.

Thanks, now I gots more stuff to brag about.