# Tag Info

1

Actually there is a geometry that describes something like the naive idea of the Big Bang. But it's a bit of a cheat because it's really just a piece of the usual expanding universe metric. The first metric suggested to describe a collapsing star was the Oppenheimer-Snyder metric, which describes a spherical ball of dust collapsing under its own gravity. ...

-1

Gravity and dark energy would have to be one & the same, due to the fact that gravity is spacetime and space time is basically the mother board or rather the rawness or better, the naked form of the universe. If einstein is correct about spacetime curvature than both are just different movements of the same entity, sort of like, how a river moves ...

1

You are describing a spherical wavefront i.e. light radiating outwards from a point source with spherical symmetry. But suppose you have two such point sources near enough to each other that their wavefronts overlap. Now your expansion model has to have space expanding simultaneously in opposite directions. Consider also that a wavefront can be any shape. ...

1

Philo's answer is spot on, and I'll basically be rephrasing it here into a form that makes more sense to me. Hopefully it will help some others as well. Rather than only dialing back the clock 1B years, let's go waaaay back and see what things look like: we go back 13.82B years and look out into space... And there's no space! The universe is very ...

2

We cannot see anything closer than 380,000 years after the big bang because that is when radiation and matter decoupled. The CMB is a picture of what the universe looked like at that point. All clumping of matter into stars, galaxies, etc has occurred since then. If we had looked 1 billion years ago, we would see the same except that the CMB temperature ...

3

In a static universe it would indeed be true that if you looked at an object, say, 10 billion light years away you would be looking at it as it had been 10 billion years ago. This isn't really an application of special relativity and is merely a consequence of a finite speed of light. Our universe, however, is expanding and so you can actually see across ...

2

just try an online calculator like Wolfam : 13.65 billion years after the big bang redshift = 0.00474 time ago (lookback time) = 65.6 million years distance (comoving) = 65.7 million ly (light years) = 20.2 Mpc (megaparsecs) = 6.22×10^20 km (kilometers) = 3.86×10^20 miles fraction of total observable radius = 0.00141 scale factor = 0.995 × current ...

3

I think I should go from another direction. Yes, obviously the Hubble constant refers to intergalactic motion, and cannot be properly applied to intragalactic effects. That does not necessarily mean that such effects do not exist (it just means that they are too minor to measure, and/or usually overshadowed by other effects; that said, I believe that GPS is ...

13

Shortly, no, this is not correct. Here's why. Hubble's law gives us that for a distance of one megaparsec, that space expands by approximately 70 km/s (the data varies, but it's somewhere between 60-80 km/s - it doesn't matter, and you'll see why). Now, how tall is your average human? Let's be generous and say your time traveler is 2m tall. Now, how many ...

14

No, because Hubble expansion has negligible effects on very small systems (such as human beings). Here is an answer which explains the maths behind it : Can the Hubble constant be measured locally??

0

And if you still need an answer for your speed of expansion (of the universe, because the solar system isn't expanding just by itself), it is measured to be about 74.3 km/s per megaparsec (a megaparsec being about 3 million light-years)

0

Our solar system isn't expanding, because it's bound by gravity. Even though space is expanding, the positions of objects in the solar system stay the same because gravity pulls them back. This is true for all gravitationally bound objects, even galaxies and galaxy clusters.

2

One way to see this is simply to note the value of the Hubble constant, about $70\rm\,km/(s\cdot Mpc)$. The Milky Way galaxy is about 0.030 Mpc in diameter, so the Hubble "flow" from one end of the Milky Way to the other is only about 2 km/s. This is much smaller than the "peculiar" motions of objects within the galaxy. Heck, the eastward motion of ...

1

In Stephen Weinberg's well-known book "The First Three Minutes," he talks solely about Doppler shifts. A good review and analysis of the "expanding space" vs. Doppler shift question was provided by Bunn and Hogg, "The kinematic origin of the cosmological red shift," Am. J. Phys. 77 (8), 2009, pp. 688-694. They make convincing arguments that the red shift ...

0

There are two parts to understanding this: Space is expanding - we know this to be true because as distance increases, so does recessional velocity (Hubble's law). So the more space in-between, the faster it recedes. Therefore the space itself is responsible for the recessions, and thus must be expanding. There is simply so much evidence for Hubble's law, ...

0

You are presumably thinking of the type of wormhole, much beloved of science fiction writers, that links two distant parts of our universe. The trouble is that we have to theory to explain how such wormholes may arise or to describe the properties of such wormholes. The nearest is the Morris-Thorne geometry, but this links two asymptotically flat regions of ...

3

The current accepted cosmological model is called $\mathrm{\Lambda{}CDM}$, and one of its basic assumptions is that the universe is isotropic. This assumption is refered to as Cosmological principle, and if true, then the rate of expansion must be equal in every direction (the "bubbles" you speak of exist at galactic scales, but not at cosmological scales: ...

1

In the Friedmann Lemaitre Roberston Walker universe, the space-time is expanding expand equally in every direction of space. Cosmological studies suggest that the universe is flat and it is believed that matter structures have grown from the stage when they were in fact small deviations from local equilibrium (they were local instabilities) and by gravity ...

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