Dark Energy, Universe Expansion And Redshift

[Eli]

In 1929 Edwin Hubble examined how the wavelength of light emitted by distant galaxies shifted towards the red end of the electromagnetic spectrum as it travels through space. He found that, galaxies were receding from each other, and fainter more distant galaxies showed a large degree of redshift than closer galaxies. With the exception of very few nearby galaxies, e.g., Andromeda, a redshift is observed in the spectra of galaxies -- most galaxies are moving away from us, and their escape velocity increases proportionally with distance. Astronomers have theorized that the fast expansion rate of the Universe is due to a mysterious form of energy, called dark energy that is constantly pulling galaxies apart.

The rate of evolutionary infinite expansion of the Universe can be described by the Einstein's general theory of relativity
\begin{equation}\tag{1}
G_{\mu \nu} + \Lambda_{\mu \nu} = \frac{8\pi G}{c^{4}}T_{\mu \nu}
\end{equation}
via the Friedmann equation
\begin{equation}\tag{2}
\Bigg(\frac{\dot{a}}{a}\Bigg)^{2} - \frac{8\pi G}{3}\rho - \frac{\Lambda c^{2}}{3} = -\frac{kc^{2}}{a^{2}}.
\end{equation}
The terms in the Friedmann equation (2), from left to right, respectively, represent the acceleration rate, density, dark energy and shape/curvature of the Universe. Dark energy is very strong than anything that we know, it is a function of vacuum, and keeps getting stronger and stronger as time passes by.

Measuring the expansion of the Universe requires the knowledge of redshift-distance relationship. The redshift and physical distances are connected by the expansion history of the Universe. Such history is coded within the redshifted CMB and the light photons emitted by the neutral hydrogen atom (HI) which reach us today after traveling million of years through space from a distant past. Redshift-cosmological distance relations enable us to extrapolate the future expansion of the cosmic by simply measuring the past expansion history of the Universe. In contrast to the distance which is the amount of physical space a photon travels through, redshift is the amount a Universe expanded during a photon's journey.

Distance measurements is one of the toughest challenges in astronomy, especially if objects are very far away from us. This is because the Universe has been expanding since these objects emitted light, and the time the light from them took to reach us today is equivalent to millions of light-years. The measurement challenge becomes even more complex due to the fact that far away objects are very faint. The technique currently used is to use magical sources of light with known true intrinsic brightness, called standard candles/rulers to track objects as the Universe expands and approximate their distances by seeing how faint they look to us.

[Eli]

https://www.forbes.com/sites/startswith ... e-universe