The Standard Big Bang Model

The Standard Big Bang Model

People slowly started to acknowledge that the earth was not the center of the solar system, let alone the Milky Way Galaxy or the Universe as a whole, and mathematicians, physicists, and astronomers were able to theoretically describe the empirical models that existed of larger and larger portions of the observable Universe. With the Messier catalog (re-published in Messier & Niles 1981) and Herschel’s Catalogue of Nebulae (Herschel, 1786) in the late eighteenth century and later Dreyer’s New General Catalog (Dreyer, 1888), it became evident that the sky was filled with objects, that were not ‘just’ stars. Though several people had speculated it that these objects were ‘island’ Universes similar to our own Milky Way at extreme distances, it was not until the work of Hubble in the early 1920s (less than 100 years ago!) that it was finally shown that they were extragalactic. 

Cepheid’s relationship

In 1925, Hubble used Cepheid’s relationship between period and absolute magnitude for the first time to show that some of the significant known nebulae, i.e., galaxies, were indeed extragalactic (Hubble 1925a, b). In Hubble (1926), he showed that extragalactic objects were spread all over the sky and derived the first empirical relations for these objects. Hubble showed that these extragalactic galaxies were all receding from the Milky Way with a velocity strongly dependent on their distance from us (Hubble, 1929). 

Hubble’s law:

This relationship can be formulated in Hubble’s law:

 v = H0 × d (1.1)


         H0 is called Hubble constant (at present indicated by 0), 

            v the receding velocity of the galaxy 

            d the distance to the galaxy.

 Thirteen years earlier, Einstein presented his general theory of relativity) generalizing the particular relativity theory and Newton’s laws of gravitation via his field 4 Introduction equations (Einstein, 1915) provides a unified theory for energy behavior, i.e., mass in the spacetime continuum. In Einstein (1917), this theory was put into a cosmological context, and how to obtain stable solutions to the field equations was discussed. Einstein created stable solutions by introducing the cosmological constant, Λ, to keep the Universe (artificially) static. Einstein later regretted having introduced Λ. However, this cosmological constant became essential to our understanding of the Universe in the 1980s. De Sitter (de Sitter 1917, 1918) and Friedmann (Friedmann 1922, 1924) later showed that stable solutions to Einstein’s field equations existed for expanding and stationary Universe models without introducing the artificial Λ. Hubble was aware of this work in 1929 and therefore noted that the linear distance-velocity relationship from Equation (1.1) might be a first approximation to the actual movement and behavior of the extragalactic nebulae. Hubble’s law is indeed model dependent at cosmological distances, but the Hubble expansion in Equation (1.1) is nevertheless a fundamental feature of the local Universe. Having established both theoretically and observationally in the 1920s that the Universe was expanding, it became apparent that in the very distant past, the Universe must have been much denser than it is today. In fact, with enough time available, it would have been considered a physical singularity, i.e., it would have been so dense and hot that all physical models would break down. Since the early 1950s, this singularity has been known as the Big Bang. The time needed for the Universe to evolve from this singularity into a Universe with the observed expansion rate is obtained by inverting Hubble’s law in Equation (1.1). 

Approximate age of the Universe

This results in an approximate age of the Universe, called the Hubble time t0 ≡ 1 H0 = d v ∼ 13.7 Gyr. (1.2) 

through the 40s and 50s, people like Gamow, Dicke, Alpher & Herman speculated that this Big Bang over-density should be observable as a smooth background black-body radiation, and they estimated it to have a temperature of the order 10s of Kelvin. 

Cosmic Microwave Background (CMB)

In 1965 the cosmic microwave background (CMB) was serendipitously observed for the first time as a spurious 3.5 K background, as presented in Penzias & Wilson (1965) and Dicke et al. (1965). This was the second observational evidence (Hubble’s distance-velocity relation in Equation (1.1) being the first) of the Big Bang model of the Universe. With the Cosmic Background Explorer (COBE; Mather et al., 1990; Smoot et al., 1991; Bennett et al., 1996) and Wilkinson Microwave Anisotropy Probe (WMAP; Hinshaw et al., 2009; Jarosik et al., 2007; Bennett et al., 2003; Jarosik et al., 2011) the CMB has later been observed in fantastic detail and has backed up these initial findings. Figure 1.2 shows the CMB maps from the COBE 4 years data (left) and the WMAP 5-year data (right).


 The cosmic microwave background (CMB) from the COBE 4-year data (left; Bennett et al., 1996) and WMAP 5-year data (right; Hinshaw et al., 2009). The color scheme depicts the 13.7 Gyr old temperature (density) fluctuations, which after inflation, grew via hierarchical merging and condensation into the galaxies we know and love. This radiation is leftover from the singularity we call the Big Bang and is one of the strongest observational proofs of the standard Big Bang model. 

Previously it had been a challenge to explain the high abundance of metals (primarily He) in the local Universe via Hydrogen burning in stars only, so when Hoyle & Tayler (1964) 

Some frequently asked question about universe and CMB

  1. What is our place in the universe?

Our place in the universe refers to our location and significance within the vast cosmos. As inhabitants of Earth, we are part of the solar system, which is just one of billions of star systems in the Milky Way galaxy. The Milky Way, in turn, is one of many galaxies in the observable universe.

  1. How did our universe come into existence?

The origin of the universe is explained by the Big Bang theory. According to this theory, the universe began as an incredibly hot and dense point roughly 13.8 billion years ago. It rapidly expanded, and over time, matter, energy, and space emerged, leading to the formation of galaxies, stars, and planets.

  1. How vast is the universe?

The universe is unimaginably vast. Its observable size is estimated to be about 93 billion light-years in diameter, which means light from the edge of the observable universe takes 93 billion years to reach us.

  1. Are we alone in the universe? The question of whether there is extraterrestrial life remains unanswered. As of now, there is no definitive evidence of life beyond Earth. However, the vastness of the universe and the discovery of exoplanets (planets outside our solar system) suggest that the possibility of life elsewhere cannot be ruled out.
  2. What is the significance of Earth in the universe? Earth is significant to us because it is our home and the only known planet to support life. It provides the perfect conditions for the existence of a diverse range of living organisms, including humans. However, on a cosmic scale, Earth is just one small planet among countless others.
  1. What is the Cosmic Microwave Background (CMB)?

The CMB is a faint microwave radiation glow that fills the universe. It is like a leftover signal from the Big Bang, which happened when the universe was born.

  1. How was the Cosmic Microwave Background discovered?

Scientists found the CMB by accident in 1965. They were using a special antenna and noticed a constant background noise coming from all directions, which turned out to be CMB radiation.

  1. Why is the Cosmic Microwave Background necessary?

The CMB is crucial because it supports the Big Bang theory. It shows us that the universe started as a hot and dense state and has been expanding since then.


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