Hubble's Law and the Expansion of the Universe

The Hubble law states that the more distant objects recede from the Milky Way galaxy at larger speeds than more nearby objects. The precise relationship is V = H_o x D, where H_o is known as the Hubble constant and is a measure of how long it takes the Universe to roughly double in size (actually to e-fold in size). The currently accepted value for the Hubble constant is ~ 73 km s^-1 per million parsecs, although there is uncertainty surrounding this number (the Hubble Tension, more on this later on).

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Interpretation of the Hubble Law

The current explanation for Hubble's Law is shown in the above box where we see galaxies and light drawn (placed) on an expanding Universe. The expansion naturally streteches the wavelengths of light leading to the redshift and. as argued now, leads to all observers in the Universe seeing an uniform expansion.

Explosion A natural explanation is that an explosion occurred and that we are at the center of the Universe. Consequently, the galaxies are flying away from us in all directions and that the ones with larger initial kicks are now farther off.

Uniform Expansion The current preferred explanation is that the Universe expands uniformly (with no center). In this scenario, any observer in the Universe sees essentially the same thing and all observers find the same Hubble's Law!

Initially, the choice was not obvious as to which of the above was correct. However, by this time people were loathe to believe that we occupied a preferred spot in the Universe. The Hubble law was interpreted as an indication of the uniform expansion of the Universe.

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Age of the Universe

• If we imagine that the Universe has been expanding at a constant rate, we can deduce roughly how long ago the objects in the Universe were sitting on top of each other (and thus the age of the Universe). If an object at distance D is moving away from us at speed V, then it is easy to figure out how long the object has been running away from us. We have

• speed = distance / time ===> time = Age of Universe ~ D / V = 1 / H_o

• Given our best measurements for the Hubble constant, H_o, we find an age for the Universe of ~ 13.7 billion years!!

  This age estimate has the potential of being annoying since there are stars which may be older than this.

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  Consider the ancient star clusters known as globular clusters:

  Globular clusters are star clusters composed of tens of thousands of stars to millions of stars. They are found in the halo of our Galaxy and represent among the oldest of the known stars in our Galaxy. Using stellar evolutionary arguments (see the Hertzsprung-Russell diagram to the right (and Astronomy 122), we can infer the ages of the stars in a globular cluster.
  globular clusters may be as old as 14 - 16 billion years (based on our understanding of stellar evolution). The oldest white dwarf stars in our Galaxy are 12-13.4 billion years old.

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  Return to Copernican Principle