21.8 Hubble's Law

21.8 Hubble's Law

Edwin Hubble established the foundation for modern cosmology by demonstrating that galaxies are moving away from the Earth, and their velocity increases with distance. In other words, the farther a galaxy is, the faster it appears to be moving away. This observation led to Hubble's law, which states that:

The recession velocity $v$ of a galaxy is directly proportional to its distance $d$ from the Earth. Therefore $v \propto d$ or

Here $v$ is the recession velocity and $d$ is the distance. $H_{o}$ is called Hubble's constant and its value is $2.3 \times 10^{-18}\,\mathrm{s^{-1}}$. Fig. 21.6 illustrates graphical representation of Hubble's law, revealing a direct and linear relationship between the redshift of galaxies and their distance from Earth. As the distance increases, the recession velocity also increases, providing a strong evidence for an expanding universe. The slope of this linear trend corresponds to the Hubble constant ($H_{o}$), which quantifies the rate of cosmic expansion. All the galaxies in the universe are moving away from each other. An observer in another galaxy will observe the same effect. This is happening because space itself is stretching. An inflated-balloon model is often considered by scientists to explain the Big Bang expansion, as shown in Fig. 21.7. The dots which are marked on the surface of a balloon represent the galaxies, and the skin of balloon represents four dimensional space-time. As the balloon is inflated, every dot "sees" the other dots moving away. This is similar to how galaxies move apart in space. This model helps us to understand two main ideas. First, no matter where we are in the universe, we would see galaxies moving away from us. This means the expansion is happening everywhere, and there is no special centre. Earth is not at the centre of the universe. The universe is expanding now, we can conceptually reverse time. Doing so implies: (i) in the past, galaxies were closer together. (ii) going further back, all matter was compressed into a very small, dense, and hot state. This idea forms the basis of the Big Bang Theory, which proposes that the universe began from an initial singularity about $13.8$ billion years ago and has been expanding ever since. Hubble's law provides strong observational evidence that the universe is not static but expanding, which is one of the key pillars supporting the Big Bang model. An approximate timeline for the evolution and expansion of the universe from the Big Bang till present is presented in Fig. 21.8.

(Fig. 21.6: Graphical representation of Hubble's law)

(Fig. 21.7: Analogy of the expanding universe: dots move apart as the balloon inflates)

Do you know?

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Before Hubble's discovery, most scientists thought the universe was static. Hubble's law turned that idea on its head, suggesting the universe had a beginning, leading to the Big Bang theory.

Brain Teaser

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A galaxy is moving away from us very fast. According to Hubble's Law, what does that tell us about its distance?