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Throughout history civilizations have developed unique systems for ordering and This section offers a tour of some of the astronomical ideas and models from.
Table of contents
- Breaking News Emails
- Cosmology’s early days
- Classical Astronomy
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The expansion has been documented by the redshifts observed in the spectra of galaxies. Furthermore, the radiation left over from the original fireball would have cooled with the expansion. The shape of the observed spectrum is an excellent fit with the theoretical Planck blackbody spectrum. The present best value for this temperature is 2. The spectrum of this cosmic radio noise peaks at approximately a one-millimetre wavelength, which is in the far infrared, a difficult region to observe from Earth; however, the spectrum has been well mapped by the Cosmic Background Explorer COBE , Wilkinson Microwave Anisotropy Probe , and Planck satellites.
Additional support for the big bang theory comes from the observed cosmic abundances of deuterium and helium.
Breaking News Emails
Normal stellar nucleosynthesis cannot produce their measured quantities, which fit well with calculations of production during the early stages of the big bang. Early surveys of the cosmic background radiation indicated that it is extremely uniform in all directions isotropic. Calculations have shown that it is difficult to achieve this degree of isotropy unless there was a very early and rapid inflationary period before the expansion settled into its present mode. Nevertheless, the isotropy posed problems for models of galaxy formation.
Galaxies originate from turbulent conditions that produce local fluctuations of density, toward which more matter would then be gravitationally attracted.
Cosmology’s early days
Such density variations were difficult to reconcile with the isotropy required by observations of the 3 K radiation. This problem was solved when the COBE satellite was able to detect the minute fluctuations in the cosmic background from which the galaxies formed. The very earliest stages of the big bang are less well understood. The conditions of temperature and pressure that prevailed prior to the first microsecond require the introduction of theoretical ideas of subatomic particle physics. Subatomic particles are usually studied in laboratories with giant accelerators , but the region of particle energies of potential significance to the question at hand lies beyond the range of accelerators currently available.
Fortunately, some important conclusions can be drawn from the observed cosmic helium abundance, which is dependent on conditions in the early big bang. The observed helium abundance sets a limit on the number of families of certain types of subatomic particles that can exist. The age of the universe can be calculated in several ways.
Assuming the validity of the big bang model, one attempts to answer the question: How long has the universe been expanding in order to have reached its present size? Independent estimates of stellar ages have yielded values less than this, as would be expected, but other estimates, based on supernova distance measurements, have arrived at values of about 15 billion years, still consistent, within the errors. A small minority of astronomers have developed alternative cosmological theories that are seriously pursued. The overwhelming professional opinion, however, continues to support the big bang model.
Finally, there is the question of the future behaviour of the universe: Is it open? That is to say, will the expansion continue indefinitely? Or is it closed, such that the expansion will slow down and eventually reverse, resulting in contraction? Thus, after about three minutes, the temperature had dropped to the one-billion-K range, making it possible for nuclear reactions of protons to take place and produce nuclei of deuterium and helium.
At the higher temperatures that prevailed earlier, these nuclei would have been promptly disrupted by high-energy photons.
With further expansion, the time between nuclear collisions had increased and the proportion of deuterium and helium nuclei had stabilized. After a few hundred thousand years, the temperature must have dropped sufficiently for electrons to remain attached to nuclei to constitute atoms. Galaxies are thought to have begun forming after a few million years, but this stage is very poorly understood.
Classical Astronomy
Star formation probably started much later, after at least a billion years, and the process continues today. Observational support for this general model comes from several independent directions. The expansion has been documented by the redshifts observed in the spectra of galaxies. Furthermore, the radiation left over from the original fireball would have cooled with the expansion. The shape of the observed spectrum is an excellent fit with the theoretical Planck blackbody spectrum.
The present best value for this temperature is 2. The spectrum of this cosmic radio noise peaks at approximately a one-millimetre wavelength, which is in the far infrared, a difficult region to observe from Earth; however, the spectrum has been well mapped by the Cosmic Background Explorer COBE , Wilkinson Microwave Anisotropy Probe , and Planck satellites.
Additional support for the big bang theory comes from the observed cosmic abundances of deuterium and helium. Normal stellar nucleosynthesis cannot produce their measured quantities, which fit well with calculations of production during the early stages of the big bang. Early surveys of the cosmic background radiation indicated that it is extremely uniform in all directions isotropic.
Calculations have shown that it is difficult to achieve this degree of isotropy unless there was a very early and rapid inflationary period before the expansion settled into its present mode. Nevertheless, the isotropy posed problems for models of galaxy formation. Galaxies originate from turbulent conditions that produce local fluctuations of density, toward which more matter would then be gravitationally attracted.
Such density variations were difficult to reconcile with the isotropy required by observations of the 3 K radiation.
The Great Ideas Astronomy and Cosmology on Apple Books
This problem was solved when the COBE satellite was able to detect the minute fluctuations in the cosmic background from which the galaxies formed. The very earliest stages of the big bang are less well understood. The conditions of temperature and pressure that prevailed prior to the first microsecond require the introduction of theoretical ideas of subatomic particle physics.
Subatomic particles are usually studied in laboratories with giant accelerators , but the region of particle energies of potential significance to the question at hand lies beyond the range of accelerators currently available. Fortunately, some important conclusions can be drawn from the observed cosmic helium abundance, which is dependent on conditions in the early big bang. The observed helium abundance sets a limit on the number of families of certain types of subatomic particles that can exist.
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The age of the universe can be calculated in several ways.