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How do we know that the universe is expanding?
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Einstein
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Credit:
Rex Features Ltd
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The expansion of universe was estimated from the answers gained by Einstein's famous "Theory of Relativity" in 1916, but it was 1929 when it was first proved by experimental observation. In order to figure out the rate of expansion, the distance to another galaxy and the velocity of the galaxy's line of sight were needed.
Let's think about 2 points in an expanding space. The distance ab, betweenpoint A and point B, increases with the time taken during expansion. At this moment, if there is a point C which is at twice the distance of A to B (so the distance between A and C is 2ab), the point C from A, will seem to go away with a speed twice faster than that between A and B. The reason is simple. This is caused by the double distance between A and C. If we decide the amount of distance between A and B that increased in a certain time of expansion is called "x", then the distance of AB after the expansion is ab+x. Ok? Now, the original distance between A and C was 2ab. For every 1 ab, the distance increases 1 x, so the distance between A and C after the expansion is 2ab+2x. Therefore, if you look from A, point B seems to have moved away at 1 x and point C has moved at a rate of 2x.
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Hubble
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Credit:
AstroArts
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In an expanding space, the things at a far distance will
seem to move away faster than something near. This idea was used when determining
the expansion of the universe. Hubble's Law, named after the scientist Edwin Powell
Hubble says "the galaxy's velocity, when it seems to depart from us, is relative
to its distance." This is why we needed to know the distance to another galaxy
and the way to measure the velocity towards our line of sight.
 To the animation of Hubble's Law made by Shockwave Flash
| Calculating Distance to Another Galaxy |
The distance to each galaxy can be found by comparing the galaxy's original brightness and its brightness seen from Earth.
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Cepheid variables in M100
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Credit:
AstroArts
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There is a type of stars called Cepheid Variables. Variables are stars that change their brightness after a certain time. In 1908, a person named Leavitt discovered that in the various Cepheid Variables in the small Magellan Nebula, the brighter-looking stars with more apparent magnitude have a longer variation period in magnitude. At that time, people already knew that apparent magnitude decreases with the amount of the distance squared. But these Cepheid variables are all in the same nebula, so the distances between them are similar. Therefore we can say that between the star's original brightness and the variable periods, the same relation as the apparent magnitude exists. The original brightness is called "absolute magnitude," but the stars with higher absolute magnitude have longer variable periods. If we can measure the absolute magnitude of the Cepheid variables, then, by comparing that amount with the apparent magnitude, we can calculate the distance. In this way, one factor needed to justify the expansion of the universe, the way to measure the distance to the galaxy, was proved.
| Calculating Velocity of the Galaxy |
Roughly speaking, the radial velocity towards each galaxy can be calculated by the light's Doppler effect.
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Light's Doppler effect
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Do you know what the "Doppler Effect" is? A familiar example of the Doppler effect of sound waves is the siren of an ambulance. The sound pitch of the siren is higher when it is approaching and lower when it is moving away.
It is a phenomena observed when the observer and the source of oscillations(light waves, sound waves, and so on.)are moving at relative speed, and the signal that spreads as waves is observed with different frequency than the waves produced by the oscillation.
Light also has the characteristic of traveling by waves. Therefore, if the light source is moving towards or away from the observer at relative speed, then the light is observed at a different wavelength than it was produced at; this is the Doppler effect. So, for example, if we say that a galaxy is approaching us, its lights will seem blue-ish because of the light's shorter wavelength caused by the light's Doppler effect. This is called a blue shift. On the other hand, if the galaxy is moving away from us, then that light will seem more diverted towards red than its original position. This is called red shift.
If you know the light's essential wavelength of the light discharged from the light source, then you can compare it to its wavelength observed on earth, and calculate the relative velocity observed from our galaxy.
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Spectroscopy by a prism
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Credit:
David Parker
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In the middle nineteenth century, it was recognized that every atom and ion produced a set of light with a particular wavelength. If these lights are tested with a spectroscope (this acts to separate lights into wavelengths. The triangular prism that separates the sunlight into seven spectra is famous for this), the peculiar wavelength of each atom and ion becomes visible in dark lines, and we call these "emission lines." To observe the separation of light by wavelength in this way is called "spectroscopic-observation." The combination of emission lines is particular to each atom and ion, so if we examine an emission line, we will know from which atom or ion it was released and how long it is. At the same time, we will know how much and in which direction (either red shift or blue shift) it diverges from its original wave length. The light from each star can be examined in this way.
This means that the way to calculate the velocity of the universe has been established. The velocity of any galaxy can be calculated by observing the light of the galaxy, and by calculating the original wavelength from emission lines and from the differences in the wavelengths due to the light's Doppler effect.
The result of calculating the velocity and the distance to each galaxy, made it clear that most galaxies are moving away from us and that velocity is proportional to its distance. Because of the recognition of Hubble's Law, the expansion of the universe became definite. One exception is our neighbor galaxy, the Andromeda Galaxy, which is approaching us. But this is due to the strong universal gravitation caused between the Andromeda and our Milky Way Galaxy, so this is an highly unusual case. At the same time, the galaxy pulls itself together due to its own universal gravitation, so the space inside the galaxy will not expand any more. The expansion of the universe only occurs in the structure of the universe which has a scale greater than the galaxy. We will explain later about these structures in "The Structure of the Universe."
To
the simulation of Universal Gravitation by Java
To
the simulation of The Red Shift by Java
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![]](../images/end.gif){kind=small}
Determine the Expansion
Introduction
The Universe Expansion
