# Tag Info

### Can a light signal from Earth reach a galaxy outside the Hubble Horizon?

There are various "horizons" in cosmology, and it is important to distinguish between them. This Wikipedia article describes several such cosmological horizons, two of which are the Hubble ...
• 56.4k
Accepted

### Understanding expansion of the Universe as things flying apart

The scenario: within a matter-dominated universe, you have prepared two particles with initially constant separation. In this scenario, those two particles will begin to fall toward each other. This ...
• 6,404
Accepted

### Will expanding space rupture an empty box floating in outer space

The box would only be ripped apart if dark energy is "phantom energy", which increases in energy density as the Universe expands. In that case, the destruction of the box would occur right ...
• 6,404
Accepted

### Is it true that $\dot{H}(t)\sim H(t)$, and if so, why?

$\dot H$ is not comparable to $H$, since the two have different dimensions. However, for a flat cosmology, you can use the two Friedmann equations to show that $\dot H$ is of order $-H^2$ (except for ...
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### Why is FTL travel impossible if the universe expands FTL?

The expansion of the universe is not associated with any faster-than-light motion. We sometimes speak of faster-than-light recession rates (for example, in the context of Hubble's law), but these ...
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### Understanding expansion of the Universe as things flying apart

Can the Universe be pretty much seen just as things flying away from each other [...] ? Absolutely! This is precisely what expansion means. That nearby particles are moving apart. (Technically, the ...
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### Viable values for the $K$ parameter in the FLRW metric

First, define a new time coordinate $\eta$ through integrating $$d\eta = \frac{c dt}{a(t)}.$$ This time coordinate is often called conformal time. With this coordinate, the condition for a radial, ...
• 1,014
1 vote

### Viable values for the $K$ parameter in the FLRW metric

$$c^2 d\tau^2 = c^2 dt^2 - \frac{a(t)^2}{(1-KX^2)} dX^2.$$ For this metric expression, if you set $K=1$, then the spatial surfaces are hyperspheres of radius $a$. Every position on these ...
• 6,404
1 vote

### Can a light signal from Earth reach a galaxy outside the Hubble Horizon?

Here are some more reasons a light signal from Earth must be able to reach and pass the Hubble Horizon: The total velocity of a photon is given by its peculiar velocity and its recession velocity as ...
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1 vote

### Is it true that $\dot{H}(t)\sim H(t)$, and if so, why?

Wild Feather wrote: "This leads me to believe that, for some reason, $\dot{H}$ can be considered to be of the same order as $H$." In a radiation dominated universe we have \rm H=+\frac{1}{...
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1 vote

### Increasing the view of observable universe

This won't work. To increase the observable universe for someone earth-bound, the probe needs to observe events and relay information about that event to earth. The probe may see the event before ...
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