# A postulate by [[Hermann Weyl]] that the lines of spacetime ([[geodesics]]) intersect at only one point, where time along each line can be synchronized; the behavior resembles an expanding fluid,<ref name="Longair-2009"/>{{rp|175}}
# [[general relativity]] that relates the geometry of spacetime to the distribution of matter and energy.
This combination greatly simplifies the equations of general relativity in tointo a form called the [[Friedmann equations]]. These equations specify the evolution of the [[Scale factor (cosmology)|scale factor]] the universe in terms of the pressure and density of a perfect fluid. The evolving density is composed of different kinds of energy and matter, each with its own role in affecting the scale factor.<ref>{{Cite book |last=White |first=Simon |title=Physics of the Early Universe: Proceedings of the Thirty Sixth Scottish Universities Summer School in Physics, Edinburgh, July 24 - August 11 1989 |date=1990 |publisher=Taylor & Francis Group |isbn=978-1-040-29413-0 |edition=1 |series=Scottish Graduate Series |___location=Milton |chapter=Physical Cosmology}}</ref>{{rp|7}} For example, a model might include [[baryons]], [[photons]], [[neutrinos]], and [[dark matter]].<ref name=PDG-2024>{{Cite journal |last=Navas |first=S. |last2=Amsler |first2=C. |last3=Gutsche |first3=T. |last4=Hanhart |first4=C. |last5=Hernández-Rey |first5=J. J. |last6=Lourenço |first6=C. |last7=Masoni |first7=A. |last8=Mikhasenko |first8=M. |last9=Mitchell |first9=R. E. |last10=Patrignani |first10=C. |last11=Schwanda |first11=C. |last12=Spanier |first12=S. |last13=Venanzoni |first13=G. |last14=Yuan |first14=C. Z. |last15=Agashe |first15=K. |date=2024-08-01 |title=Review of Particle Physics |url=https://link.aps.org/doi/10.1103/PhysRevD.110.030001 |journal=Physical Review D |language=en |volume=110 |issue=3 |doi=10.1103/PhysRevD.110.030001 |issn=2470-0010}}</ref>{{rp|25.1.1}} These component densities become parameters extracted when the model is constrained to match astrophysical observations.
The most accurate observations which are sensitive to the component densities are consequences of statistical inhomogeneity called "perturbations" in the early universe. Since the Friedmann equations assume homogeneity, additional theory must be added before comparison to experiments. [[Inflation (cosmology)|Inflation]] is a simple model producing perturbations by postulating an extremely rapid expansion early in the universe that separates quantum fluctuations before they can equilibrate. The perturbations are characterized by additional parameters also determined by matching observations.<ref name=PDG-2024/>{{rp|25.1.2}}
