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When we consider the origin of the universe known as

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when we consider the origin of the Universe known as the Big Bang. Due to isotropy, there is no `place' where the BigBang occurred, there is no center point. Empedocles stated it best by saying `God is an infinite sphere whose center iseverywhere and circumference nowhere.Cosmological Principle:Observations to date support the idea that the Universe is both isotropic and homogeneous. Both facts are linked to what iscalled the cosmological principle. The cosmological principle derives from theCopernican Principlebut has nofoundation in any particular physical model or theory, i.e. it can not be `proved' in a mathematical sense. However, it hasbeen supported by numerous observations of our Universe and has great weight from purely empirical grounds.
A corollary to the cosmological principle is that the laws of physics are universal. The same physical laws and models thatapplies here on the Earth also works in distant stars, galaxies, and all parts of the Universe - this of course simplifies ourinvestigations immensely. Note also that it is assumed that physical constants (such as the gravitational constant, mass ofthe electron, speed of light) are also the unchanging from place to place within the Universe, and over time.The clearest modern evidence for the cosmological principle is measurements of the cosmic microwave background(shown above). Briefly (we will cover the CMB in a later lecture), the CMB is an image of the photons emitted from theearly Universe. Isotropy and homogeneous is reflected in its random appearance.The greatest consequence of the cosmological principle is that it implies that all parts of space are causally connected atsome time in the past (although they may no longer be connected today). Thus, a homogeneous Universe leads to theconclusion that the whole Universe appeared at a single moment of time, a Creation.
Lastly, we is we extend the cosmological principle through time we have the `perfect' cosmological principle, that theUniverse is isotropic and homogeneous, and has been for all time. This means that the laws of Nature are unchanging andthat things we observe from the past can be assumed to operate under that same physics as things toady.Important Models of Universe(i)Steady State and(ii) Big Bang model(i)steady state modelIncosmology, thesteady state modelis an alternative to theBig Bangtheory of the evolution of the universe. In thesteady state model, the density of matter in theexpanding universeremains unchanged due to a continuous creation ofmatter, thus adhering to theperfect cosmological principle, a principle that asserts that theobservable universeis basicallythe same at any time as well as at any place.While the steady state model enjoyed some popularity in the mid-20th century (though less popularity than the Big Bangtheory), it is now rejected by the vast majority ofcosmologists,astrophysicistsandastronomers, as the observationalevidence points to a hot Big Bang cosmology with a finiteage of the universe, which the steady state model does notpredictCounts of radio sources

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Term
Fall
Professor
NoProfessor
Tags
General Relativity, Big Bang, Redshift, Physical cosmology, Cosmic microwave background radiation, CMB

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