Thus, each grain would have a horizon — a boundary beyond which the rest of the Universe is invisible. As Eddington put it, the region beyond “is altogether shut off from us by this barrier of time.” The concept of a cosmological horizon is essential in modern cosmology. It turns out to be the correct description of the Universe we live in. We cannot see beyond our cosmological horizon, which we now know has a radius of 46.5 billion light-years. And since no point in the Universe is central — it grows in all directions at once — other observers from other points in the Universe would have their own cosmic aquaria. Like many scientists who study dark matter, Mack says the only thing we really know about it is that there’s five times as much of it as known matter. George Lemaître, a contemporary Belgian physicist, interpreted data from Hubble and others as evidence of an expanding universe, a possibility permitted by Einstein’s recently published field equations of general relativity.

Cosmology is unlike the other sciences

(The same occurs if the observer is moving away from the light source.) If the source is approaching, the light is squeezed to shorter wavelengths, or blueshifted. So if astronomers could measure the light from distant galaxies, physicists would know whether the Universe is expanding or not.

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The very early universe is said to have been ‘radiation dominated’ and radiation controlled the deceleration of expansion. Later, as the average energy per photon becomes roughly 10 eV and lower, matter dictates the rate of deceleration and the universe is said to be ‘matter dominated’. As the expansion of the universe continues, matter dilutes even further and the cosmological constant becomes dominant, leading to an acceleration in the universe’s expansion. Different forms of energy may dominate the cosmos—relativistic particles which are referred to as radiation, or non-relativistic particles referred to as matter.

Formation and evolution of large-scale structure

These problems are resolved by a brief period of cosmic inflation, which drives the universe to flatness, smooths out anisotropies and inhomogeneities to the observed level, and exponentially dilutes the monopoles. The physical model behind cosmic inflation is extremely simple, but it has not yet been confirmed by particle physics, and there are difficult problems reconciling inflation and quantum field theory.

Ancient Greek Cosmology: How Did the Greeks See the Universe?

Following this, in the early universe, the evolution of the universe proceeded according to known high energy physics. This is when the first protons, electrons and neutrons formed, then nuclei and finally atoms. With the formation of neutral hydrogen, the cosmic microwave background was emitted. Finally, the epoch of structure formation began, when matter started to aggregate into the first stars and quasars, and ultimately galaxies, clusters of galaxies and superclusters formed. The future of the universe is not yet firmly known, but according to the ΛCDM model it will continue expanding forever.