Our most powerful telescopes can only see a certain distance into space. What lies beyond that?
Since light travels at a finite speed, all telescopes look back in time. The further away things are the younger the Universe was when the light was emitted.
Our telescopes can actually see as far as it is possible to see with electromagnetic radiation. We see the Cosmic Microwave Background, the relic radiation of the Big Bang.
When the Universe was very young it was much denser and hotter. As it expanded it cooled. The same physics happens to a gas inside an insulated cylinder if you pull a piston out. The gas cools as the available volume increases.
When the Universe was 380,000 years old it cooled enough that the average energy of electrons dropped below the energy needed to knock them out of hydrogen atoms. At earlier times no atoms could exist. The Universe was so hot that atomic matter was a plasma of free electrons, protons and helium nuclei.
Photons of light scatter from charged particles and do not travel far when trapped in a hot dense plasma. The same is true of the Sun today, which is why it is opaque. Light produced in nuclear reactions in the interior of the Sun takes around a million years to diffuse to the surface.
The whole Universe was opaque until the first hydrogen atoms formed at age 380,000 years. When that happened there were no longer any free charged particles causing the photons of light to scatter. The Universe suddenly became transparent, and the photons were able to travel unimpeded.
The wavelengths of those photons have been stretched a factor 1090 by the expansion of the Universe in the time since they were emitted. That means most of the relic radiation today is found in the microwave region of the electromagnetic spectrum. Hence the term "Cosmic Microwave Background" (CMB). It has a temperature of 2.7 degrees above absolute zero today, but would have been 1090 times hotter when the first atoms formed.
To look back further to earlier times when the Universe was more than 3000 degrees is impossible with telescopes based on electromagnetic waves.
The answer to your question of what lies beyond the CMB is then: something like the inside of the Sun when the Universe was more than a minute old. The Large Hadron Collider gives us some idea of what the Universe was like when it was tiny fractions of a second old. But the highest energies and densities cannot be probed in any lab on Earth.
We know that there is a cosmic neutrino background, and possibly also a gravitational wave background that might be detected from the very early Universe. The present temperature of these backgrounds is less than 2 degrees above absolute zero. That makes them extremely difficult to measure. It is something we might dream of doing many decades from now.
Professor David Wiltshire
School of Physical and Chemical Sciences
University of Canterbury