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Top Document: [sci.astro] Galaxies (Astronomy Frequently Asked Questions) (8/9) Previous Document: H.02.4 Searches for Dark Matter Next Document: H.04 How are galaxy distances measured? See reader questions & answers on this topic! - Help others by sharing your knowledge
Joseph Lazio <jlazio@patriot.net>
By 1925, V. M. Slipher had compiled radial velocities for 41 galaxies.
He noticed that their velocities were quite a bit larger than typical
for objects within our Galaxy and that most of the velocities
indicated recession rather than approach. In 1929, Edwin Hubble (and
others) recognized the simple relationship that recession velocity is
on average proportional to the galaxy's distance. (His distance
measure was the apparent magnitude of the brightest individually
recognizable stars.) This proportionality is now called "Hubble's
Law," and the constant of proportionality is known as the "Hubble
constant," H (often written "Ho," i.e., H subscript zero).
The Hubble constant also has the property of being related to the age
of the Universe, which undoubtedly explains some of the interest in
its value. It is a constant of proportionality between a speed
(measured in km/s) and a distance (measured in Mpc), so its units are
(km/s)/Mpc. Since kilometers and megaparsecs are both units of
distance, with the correct factor, we can convert megaparsecs to
kilometers, and we're left with a number whose units are (km/s)/km.
If we take 1/H, we see that it has units of seconds, that is 1/H is a
time. We might consider 1/H to be the time it takes for a galaxy
moving at a certain velocity (in km/s) to have moved a certain
distance (in Mpc). If the galaxies have always been moving exactly as
they now are, 1/H seconds ago all of them were on top of us!
Of course the proportionality isn't exact for individual galaxies. Part
of the problem is uncertainties in measuring the distances of galaxies,
and part is that galaxies don't move entirely in conformity with the
"Hubble Flow" but have finite "peculiar velocities" of their own. These
are presumably due to gravitational interactions with other, nearby
galaxies. Some nearby galaxies indeed have blue shifts; M 31 (the
Andromeda galaxy) is a familiar example.
In order to measure the Hubble constant, all one needs a distance and a
redshift to a galaxy that is distant enough that its peculiar velocity
does not matter. Measuring redshifts for galaxies is easy, but
measuring distances is hard. (See the next question.) The Hubble
constant is therefore not easy to measure, and it is not surprising that
there is controversy about its value. In fact, there are generally two
schools of thought: one group likes a Hubble constant around 55
(km/s)/Mpc, and another prefers values around 90 (km/s)/Mpc.
When converted to an age of the Universe, H = 55 (km/s)/Mpc corresponds
to an age of about 19 billion years and H = 90 (km/s)/Mpc is an age of
11 billion years (again if the velocities are constant).
A measure of how difficult it is to determine the Hubble constant
accurately can be seen by examining the different values reported. A
search by Tim Thompson <tim@lithos.Jpl.Nasa.Gov> for the period
1992--1994 found 39 reported values for H in the range
40--90 (km/s)/Mpc.
The linear relation between distance and recession velocity breaks down
for redshifts around 1 and larger (velocities around 2E5 km/s). The
true relation depends on the curvature of space, which is a whole other
topic in itself (and has no clear answer). The sense, though, is that
infinite redshift, corresponding to a recession velocity equal to the
speed of light, occurs at a finite distance. This distance is the
"radius of the observable Universe." Nothing more distant than this can
be observed, even in principle.
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Last Update March 27 2014 @ 02:11 PM
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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