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Welcome to our Physics lesson on The Discovery Made by Hubble, this is the third lesson of our suite of physics lessons covering the topic of Expansion of the Universe, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
In 1929, when calculating the distances of galaxies through the Cepheid method and their receding speed through the Doppler Effect formula, the English scientist Edwin Hubble discovered a stunning rule about galaxies: their receding speed is proportional to the distance from us. Mathematically, we have:
where r is the distance of a galaxy from Earth.
From mathematics, it is known that we must multiply one side of a proportionality by a constant to turn it into equation. This constant is denoted by H0 and is known as the Hubble Constant. Hence, the above relation becomes
This equation is the mathematical expression of Hubble's Law. The original value of Hubble Constant measured using the methods of that time was 500 km/(s·Mpc). However, this value is very far from the truth. The most exact value measured so far is
Hubble's discovery represents an important scientific revolution because it made scientists abandon the idea of a static, eternal and unchangeable universe. It also represents a revolutionary method in distance measurement of remote galaxies.
Assuming the Hubble Constant as unchangeable in time, calculate how many years ago the Andromeda galaxy was 1.5 million light years away from ours. Take the actual distance of Andromeda galaxy from Milky Way equal to 2.5 million light years.
First, we have to calculate the receding speed of Andromeda galaxy by applying Hubble's Law. Thus, since r = 2.5 million light years = 2.5 × 106 ly = 2.5 × 106 ly/3.26 (pc/ly) = 0.76687 × 106 pc = 0.76687 Mpc, we obtain
During the given period of time, the Andromeda galaxy has receded by s = 2.5 million light years - 1.5 million light years = 1 million light years = 106 ly. When expressed in km, this distance becomes
To find this result, we have used the conversion factor given in 22.7:
Therefore, the time elapsed since Andromeda galaxy was 1 million light years away from ours, is
Last, we have to express this distance in years. Given that 1 y = 365.2422 d = 365.2422 × 24 × 60 × 60 s = 3.1557 × 107 s, we obtain
You have reached the end of Physics lesson 22.9.3 The Discovery Made by Hubble. There are 6 lessons in this physics tutorial covering Expansion of the Universe, you can access all the lessons from this tutorial below.
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