The observable universe refers to the of the that can be seen from Earth. This volume is limited by the distance that has had time to travel since the , given the universe's age. Light from extremely distant alaxies takes of years to reach us, so we are effectively looking into the past when observe these celestial objects.

What is the observable Universe?

Why was the universe opaque for the first 380,000 years?*1 pointUnbonded electrons absorbed much of the electromagnetic radiation that was present.There was too much dust made from the earliest atoms blocking the light.No light was being emitted as there was low amounts of electromagnetic radiation.There was high levels of electromagnetic radiation, but only microwaves and radio waves which do not include visible light.

How long have we known about our place in the universe? When were the galaxies discovered?

The expansion of the universe is the increase in distance between gravitationally unbound parts of the observable universe with time.[1] It is an intrinsic expansion; the universe does not expand "into" anything and does not require space to exist "outside" it. To any observer in the universe, it appears that all but the nearest galaxies (which are bound to each other by gravity) recede at speeds that are proportional to their distance from the observer, on average. While objects cannot move faster than light, this limitation only applies with respect to local reference frames and does not limit the recession rates of cosmologically distant objects.Cosmic expansion is a key feature of Big Bang cosmology. It can be modeled mathematically with the Friedmann–Lemaître–Robertson–Walker metric (FLRW), where it corresponds to an increase in the scale of the spatial part of the universe's spacetime metric tensor (which governs the size and geometry of spacetime). Within this framework, the separation of objects over time is associated with the expansion of space itself. However, this is not a generally covariant description but rather only a choice of coordinates. Contrary to common misconception, it is equally valid to adopt a description in which space does not expand and objects simply move apart while under the influence of their mutual gravity.[2][3][4] Although cosmic expansion is often framed as a consequence of general relativity, it is also predicted by Newtonian gravity.[5][6]According to inflation theory, during the inflationary epoch about 10−32 of a second after the Big Bang, the universe suddenly expanded, and its volume increased by a factor of at least 1078 (an expansion of distance by a factor of at least 1026 in each of the three dimensions). This would be equivalent to expanding an object 1 nanometer (10−9 m, about half the width of a molecule of DNA) in length to one approximately 10.6 light years (about 1017 m or 62 trillion miles) long. Cosmic expansion subsequently decelerated to much slower rates, until at around 9.8 billion years after the Big Bang (4 billion years ago) it began to gradually expand more quickly, and is still doing so. Physicists have postulated the existence of dark energy, appearing as a cosmological constant in the simplest gravitational models, as a way to explain this late-time acceleration. According to the simplest extrapolation of the currently favored cosmological model, the Lambda-CDM model, this acceleration becomes more dominant into the future.

How did Hubble know that the galaxies he observed were moving away from us? aThe spectra of the stars were red-shifted which indicates they were moving away rapidly. bThe spectra of the stars were blue-shifted which indicates they were moving away rapidly. cThe spectra of the stars were violet-shifted which indicates they were moving away rapidly. dThe spectra of the stars were green-shifted which indicates they were moving away rapidly.