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In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:
X-ray astronomy is an observational branch of astronomy which deals with the study of X-ray observation and detection from astronomical objects. X-radiation is absorbed by the Earth's atmosphere, so instruments to detect X-rays must be taken to high altitude by balloons, sounding rockets, and satellites. X-ray astronomy uses a type of space telescope that can see x-ray radiation which standard optical telescopes, such as the Mauna Kea Observatories, cannot.
In astronomy, the interstellar medium (ISM) is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field.
Outer space is the expanse that exists beyond Earth and between celestial bodies. Outer space is not completely empty—it is a hard vacuum containing a low density of particles, predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, neutrinos, dust, and cosmic rays. The baseline temperature of outer space, as set by the background radiation from the Big Bang, is 2.7 kelvins. The plasma between galaxies is thought to account for about half of the baryonic (ordinary) matter in the universe, having a number density of less than one hydrogen atom per cubic metre and a temperature of millions of kelvins. Local concentrations of matter have condensed into stars and galaxies. Studies indicate that 90% of the mass in most galaxies is in an unknown form, called dark matter, which interacts with other matter through gravitational but not electromagnetic forces. Observations suggest that the majority of the mass-energy in the observable universe is dark energy, a type of vacuum energy that is poorly understood. Intergalactic space takes up most of the volume of the universe, but even galaxies and star systems consist almost entirely of empty space.
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The missing baryon problem arises from a mismatch between the amount of baryonic matter detected shortly after the Big Bang and at late times. Over the last decades, observations of the cosmic microwave background and Big Bang nucleosynthesis studies have set strict constraints on the abundance of baryons in the early universe, finding that baryonic matter accounts for 4.85% of the energy contents of the Universe. At the same time, census of baryons in the late Universe have found that all the mass in planets, stars, interstellar gas, and gas surrounding galaxies only accounts for less than half of the baryons detected in the early Universe. This strong discrepancy is commonly known as the missing baryon problem. The missing baryon problem is distinct from the dark matter problem, which is non-baryonic in nature.
References
- 1 2 "H2356-309: X-ray Discovery Points to Location of Missing Matter". Chandra. Retrieved 28 November 2019.
- 1 2 3 "QSO B2356-309". Simbad. Retrieved 27 November 2019.
- ↑ Fang, Taotao (2010). "Confirmation of X-Ray Absorption by Warm-Hot Intergalactic Medium in the Sculptor Wall". The Astrophysical Journal. 714 (2): 1715–1724. arXiv: 1001.3692 . Bibcode:2010ApJ...714.1715F. doi:10.1088/0004-637X/714/2/1715. S2CID 17524108.
