Current Theoretical Understanding

The Four Forces

All interactions in the Universe are believed to be governed by four fundamental forces: gravity (the attraction between two objects that have mass or energy), the weak force (responsible for particle decay), the strong force (holds quarks, building blocks of protons and neutrons, together) and the electromanetic force (acts between charged particles, like negatively charged electrons and positively charged protons). These forces are believed to be given rise by exchanges of fundamental particles, known as "force carriers", which are themselves quanta of energy in a particular kind of field. For the electromagnetic force it's photons, strong force - gluons, weak force - W and Z bosons. Graviton is the hypothetical force carrier of gravity, but no complete quantum field theory of gravitons has been produced.

Relativity

In early 20th century, classical Newtonian mechanics were discarded in favour of Einstein's theory of relativity. He determined that the laws of physics are the same for all non-accelerating observers, and that the speed of light in a vacuum was independent of the motion of all observers. This was the theory of special relativity. It introduced a new framework for all of physics and proposed new concepts of space and time. In 1915, Einstein proposed his theory of general relativity, where he determined that massive objects cause a distortion in space-time, which is felt as gravity.

Quantum

Quantum mechanics arose gradually from theories to explain observations which could not be reconciled with classical physics, such as Max Planck's solution in 1900 to the black-body radiation problem, and the correspondence between energy and frequency in Albert Einstein's 1905 paper which explained the photoelectric effect, and light was discovered to be quantized. The features of the theory include wave-particle dualiity, wave functions probability amplitudes, quantum entanglement and many many more. It has not yet been reconciled with Einstein's relativity, even though both theories are experimentally verified, and many physicists are looking for a grand unified theory.

Technology

Telescopes

Modern telescopes have gone far beyond those constructed by Newton and Galilei. They gather information from the electromagnetic spectrum far beyond the range of visible light. Telescopes that survey radio, x-ray, and gamma-ray wavelength have dramatically broadened our understanding of the universe. Radio telescopes have helped to map the spiral arms of our galaxy, while gamma-ray observatories high in Earth orbit have captured the high-energy signals of black holes and gamma-ray bursts.

Observatories

Ground-based observatories are used to make observations in the radio and visible light portions of the electromagnetic spectrum. Most optical telescopes are housed within a dome or similar structure, while radio telescopes do not have domes. Since the mid-20th century, a number of astronomical observatories have been constructed at very high altitudes, above 4000–5000 m.

Space Telescopes

The 1st operational space telescopes were the American OAO-2 launched in 1968, and the Soviet Orion 1 ultraviolet telescope in 1971. Space telescopes avoid the distortion of electromagnetic radiation as well as light pollution. This makes them a lot more effective in observing even the most distant corners of the universe. In 1990, the Hubble ST was launched into orbit. It is one of the largest and most versatile telescopes, observing in the ultraviolet, visible, and near-infrared regions of the electromagnetic spectrum.

Ongoing Research

Dark Matter

Galaxies in our universe are rotating with such speed that the gravity generated by their observable matter could not possibly hold them together. This leads scientists to believe that something we have yet to detect directly is giving these galaxies extra mass, generating the extra gravity they need to stay intact. This unknown matter was called “dark matter” since it is not visible. Unlike normal matter, dark matter does not interact with the electromagnetic force. This means it does not absorb, reflect or emit light, making it extremely hard to spot. Researchers have been able to infer the existence of dark matter only from the gravitational effect it seems to have on visible matter.

Dark Energy

In 1998, the Hubble Space Telescope observations of distant supernovae indicated that the Universe was expanding at an increasing rate. This contradicted the theoretical prediction that, as time went on, expansion would slow down due to the force of gravity. There are no concrete answers, but the working theory is that it is dark energy, an elusive new type of energy, that is causing this acceleration of expansion. It is extimated to comprise 68% of the universe, however no one knows where it comes from.

Exoplanets

Exoplanets are planets beyond our solar system. Most orbit other stars, but free-floating exoplanets, called rogue planets, orbit the galactic center and are untethered to any star. Since 1990s, thousands of exoplanets have been discovered by NASA’s Kepler Space Telescope. By measuring exoplanets’ sizes and masses, we can see compositions ranging from very rocky (like Earth and Venus) to very gas-rich (like Jupiter and Saturn). Most exoplanets are found through indirect methods: measuring the dimming of a star that happens to have a planet pass in front of it, called the transit method, or monitoring the spectrum of a star for the tell-tale signs of a planet pulling on its star and causing its light to subtly Doppler shift. Those planets positioned in the habitable zones of their stars have the possibility of harbouring life, and are thus an exciting area of research for astronomers.