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Top Document: [sci.astro] Cosmology (Astronomy Frequently Asked Questions) (9/9) Previous Document: I.06. What is inflation? Next Document: I.08. If the Universe is only 10 billion years old, how can we see objects that are now 30 billion light years away? Why See reader questions & answers on this topic! - Help others by sharing your knowledge
(Also, can objects expand away from us faster than the speed
of light?)
In the Big Bang model the *distance* between galaxies increases, but
the galaxies don't move. Since nothing's moving, there is no
violation of the restriction that nothing can move faster than light.
Hence, it is quite possible that the distance between two objects is
so great that the distance between them expands faster than the speed
of light.
What does it mean for the distance between galaxies to increase
without them moving? Consider two galaxies in a one-dimensional Big
Bang model:
*-|-|-|-*
0 1 2 3 4
There are four distance units between the two galaxies. Over time the
distance between the two galaxies increases:
* - | - | - | - *
0 1 2 3 4
However, they remain in the same position, namely one galaxy remains
at "0" and the other remains at "4." They haven't moved.
(Astronomers typically divide the distance between two galaxies into
two parts, D = a(t)*R. The function a(t) describes how the size of
the Universe increases, while the distance R is independent of any
changes in the size of the Universe. The coordinates based on R are
called "co-moving coordinates.")
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Last Update March 27 2014 @ 02:11 PM
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[sci.astro] Cosmology (Astronomy Frequently Asked Questions) (9/9)
Section - I.07. How can the Big Bang (or inflation) be right? Doesn't it violate the idea that nothing can move faster than light?
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[sci.astro] Cosmology (Astronomy Frequently Asked Questions) (9/9)
Section - I.07. How can the Big Bang (or inflation) be right? Doesn't it violate the idea that nothing can move faster than light?
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with stars, then every direction you looked would eventually end on
the surface of a star, and the whole sky would be as bright as the
surface of the Sun.
Why would anyone assume this? Certainly, we have directions where we look that are dark because something that does not emit light (is not a star) is between us and the light. A close example is in our own solar system. When we look at the Sun (a star) during a solar eclipse the Moon blocks the light. When we look at the inner planets of our solar system (Mercury and Venus) as they pass between us and the Sun, do we not get the same effect, i.e. in the direction of the planet we see no light from the Sun? Those planets simply look like dark spots on the Sun.
Olbers' paradox seems to assume that only stars exist in the universe, but what about the planets? Aren't there more planets than stars, thus more obstructions to light than sources of light?
What may be more interesting is why can we see certain stars seemingly continuously. Are there no planets or other obstructions between them and us? Or is the twinkle in stars just caused by the movement of obstructions across the path of light between the stars and us? I was always told the twinkle defines a star while the steady light reflected by our planets defines a planet. Is that because the planets of our solar system don't have the obstructions between Earth and them to cause a twinkle effect?
9-14-2024 KP