The big bang theory is supported by space’s expansion, cosmic microwave background radiation, and the abundance of elements in the universe shortly after the big bang. Not only does each piece of evidence individually point towards the big bang, but the multiple sources of backing for this theory are also able to tie together in their support for it.
The expansion of space is one widely supported line of evidence in favor of the big bang theory that is demonstrated by observable redshifts and Hubble’s law. According to the theory, it is possible to directly detect space’s expansion through redshift by examining the changes in wavelengths of light from distant stars and galaxies. To start off, many people are familiar with the Doppler effect with respect to sound waves; for example, if an ambulance passes by, an observer will notice a change in the siren’s pitch as it approaches and moves away. This difference in frequency and wavelength is related to the motion of the ambulance. When the vehicle is approaching, the sound is moving in the same direction as the ambulance relative to the observer. This compresses the sound waves, shortening the wavelength, increasing the frequency, and increasing the detected pitch of the siren. Consequently, when the ambulance is moving away, the sound waves are stretched out, which causes the observer to perceive a lower pitch, marking a decrease in frequency and an increase in wavelength.
This idea can be applied to light waves as well. Stars typically give off different wavelengths of light depending on the different elements being fused. The fusion of each element yields a different wavelength of light, giving each type of star a unique spectral pattern, known as characteristic spectral lines. If a star or galaxy is moving away from the earth, the light waves that it emits will be stretched out as they reach us, and the wavelength will be longer. This translates into a shift in the star’s spectral lines towards the red (low frequency) end of the spectrum. Simply put, “an approaching light source is seen as blue shifted as it approaches, and is red shifted as it moves away,” according to Luciuk. These redshifts are direct proof that the universe is expanding and that other galaxies are moving away from us.
Hubble’s law relates the redshifts observed on earth to expansion on a larger scale, stating that there is a direct correlation between the distance from a galaxy and the velocity of its receding. The law itself is a mathematical relationship between distance and recessional velocity that can be used to determine the expansion of galaxies around us. Scientists have been able to extrapolate from nearer galaxies’ behavior that the further ones are moving away at higher speeds.
This result obtained from Hubble’s law is also indicative of the nature of the expansion itself; it is characteristic of a homogeneous, isotropic expansion of the universe. This means that from any given point of observation in space, the surrounding galaxies appear to be moving away. According to Kolb, “If the big-bang model is correct, then any observer on any galaxy anywhere in the universe would see galaxies receding… The expansion of the universe is an expansion of space, not an expansion of galaxies into space.”
Cosmic Microwave Background Radiation
Aside from space’s expansion, a second convincing piece of evidence for the big bang is the presence of cosmic microwave background radiation, or “ancient photons” that are remnants of the early universe. Directly after the bang, the universe was much more dense than today and had a much higher temperature. As a result of these conditions, subatomic particles could not yet combine to form atoms, and the presence of electrons caused a scattering of photons, which should possess the same frequency as the cosmic microwave background. In fact, this was accurately measured by NASA’s COBE satellite, with negligible differences between the actual and theoretical energy spectrums. The cosmic background radiation can be detected from multiple directions (without an observable source), and its distribution is extremely even throughout the universe.
The relative abundance of elements in the early universe is also evidence for the big bang when compared with abundances observed in certain starstoday. During the early phases of the universe, the high temperatures allowed for fusion of lighter nuclei to form heavier elements, known as the process of nucleosynthesis. By understanding the conditions of the early universe like temperature and density of matter, it is possible to predict the relative abundances of the first elements formed (namely hydrogen, helium, and lithium). These can then be compared with the abundance of elements in old stars to test the accuracy of the theory. Since certain stars, such as quasars, have a significant red shift in their spectral lines, it can be inferred that they are very far away and that we are observing them in an early development stage a long time ago. We can then use their spectral lines to determine the amounts of various elements being fused in the star and compare these with the theoretical values generated by the big bang theory. Astronomers found that the abundance of deuterium, an unstable isotope of hydrogen, in ancient stars lined up favorably with the predictions established by the big bang theory.
Overall, the expansion of space, cosmic background radiation, and abundance of light elements all provide compelling evidence in favor of the big bang theory.
Facts for Kids About the Big Bang Theory
- The model postulates that at some moment all of space was contained in a single point from which the universe has been expanding ever since.
- Giant clouds of these primordial elements later coalesced through gravity to form stars and galaxies.
- The Big Bang theory does not provide any explanation for the initial conditions of the universe; rather, it describes and explains the general evolution of the universe going forward from that point on.
- Since Georges Lemaître first noted, in 1927, that an expanding universe might be traced back in time to an originating single point, scientists have built on his idea of cosmic expansion.
- In 1964, the cosmic microwave background radiation was discovered, which was crucial evidence in favor of the Big Bang model, since that theory predicted the existence of background radiation throughout the universe before it was discovered.
- The known physical laws of nature can be used to calculate the characteristics of the universe in detail back in time to an initial state of extreme density and temperature.
- Though simple atomic nuclei formed within the first three minutes after the Big Bang, thousands of years passed before the first electrically neutral atoms formed.
- Giant clouds of these primordial elements later coalesced through gravity to form stars and galaxies, and the heavier elements were synthesized either within stars or during supernovae.
- Large particle accelerators can replicate the conditions that prevailed after the earliest moments of the universe, resulting in confirmation and refinement of the details of the Big Bang model.
- The framework for the Big Bang model relies on Albert Einstein’s theory of general relativity and on simplifying assumptions such as homogeneity and isotropy of space.
- Based on measurements of the expansion using Type Ia supernovae, measurements of temperature fluctuations in the cosmic microwave background, and measurements of the correlation function of galaxies, the universe has an estimated age of 13.798 ± 0.037 billion years.
- After about 10 seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in particle physicsexperiments.
- A few minutes into the expansion, when the temperature was about a billion kelvin and the density was about that of air, neutrons combined with protons to form the universe’s deuterium and helium nuclei in a process called Big Bang nucleosynthesis.
- As the universe cooled, the rest mass energy density of matter came to gravitationally dominate that of the photon radiation.
- Independent lines of evidence from Type Ia supernovae and the CMB imply that the universe today is dominated by a mysterious form of energy known as dark energy, which apparently permeates all of space.
- The Big Bang theory depends on two major assumptions: the universality of physical laws and the cosmological principle.
- Cosmologists now have fairly precise and accurate measurements of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the universe appears to be accelerating.
- Measurements of the cosmic microwave background indicate that the universe is very nearly spatially flat, and therefore according to general relativity the universe must have almost exactly the critical density of mass/energy.