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Physics Reference Libraries

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Quintessence Dark Energy
2004-10-19 04:45:41

Quintessence (Dark Energy) -- Quintessence or dark energy is a hypothetical form of energy postulated to exist in order to explain observations of an accelerating universe. This energy would act like a vacuum pressure, pushing things apart. Other attempts to explain these recent observations involve a non-zero cosmological constant, which has the same effect. Indeed, sometimes...

Cosmological Constant
2004-10-19 04:45:41

Cosmological Constant -- The cosmological constant (usually denoted by the Greek capital letter lambda: Λ) is a value occurring in Einstein's theory of general relativity. The units of Λ are 1/second2; its value is unknown but believed to be positive based on recent observations. The constant is proportional to the energy density of the vacuum ρ, where π is Pi, G is the...

Cosmic Rays
2004-10-19 04:45:41

Cosmic Rays -- Cosmic Rays. Cosmic rays are energetic particles that are found in space and filter through our atmosphere. Cosmic rays have interested scientists for many different reasons. They come from all directions in space, and the origination of many of these cosmic rays is unknown. Cosmic rays were originally discovered because of the ionozation they produce in our atmosphere....

Inflation
2004-10-19 04:45:41

Inflation -- Inflation is the idea - first proposed by Alan Guth (1981) - that the nascent universe passed through a phase of exponential expansion that was driven by a negative vacuum energy density (positive vacuum pressure). This expansion can be modelled by a non-zero cosmological constant. As a direct consequence of this expansion, all of the observable universe is posited to have...

Cosmic Background Radiation
2004-10-19 04:45:41

Cosmic Background Radiation -- The Big Bang theory predicts that the early universe was a very hot place and that as it expands, the gas within it cools. Thus the universe should be filled with radiation that is literally the remnant heat left over from the Big Bang, called the cosmic microwave background radiation, or CMB. When any patch of the sky is observed where no individual sources...

Corona
2004-10-19 04:45:41

Corona -- The corona is the luminous "atmosphere" of the Sun extending millions of kilometers into space, most easily seen during a total solar eclipse. An interesting feature of the corona is the fact that it is much hotter than the visible "surface" of the Sun; the photosphere is approximately 6000°C compared to the corona at over one million °C. The corona is much less dense than...

Black Hole
2004-10-19 04:45:41

Black Hole -- Black holes are objects so dense that not even light can escape their gravity. They are believed to form from the gravitational collapse of astronomical objects containing two or more solar masses. Astronomical observations suggest that the center of most galaxies, including our own Milky Way, contain supermassive black holes containing millions to billions of solar masses....

Big Bang
2004-10-19 04:45:41

Big Bang -- The Big Bang theory is the dominant theory in cosmology about the early development and current shape of the universe. According to this theory, the universe expanded rapidly starting 13.7 0.2 billion years ago. Extrapolating the history of the universe backwards using current physical models leads to a gravitational singularity, at which all distances become zero and...

Aurora
2004-10-19 04:45:41

Aurora -- The Polar Aurora are natural displays of light in the sky that can be seen with the unaided eye only at night. An auroral display in the Northern Hemisphere is called the aurora borealis, or the northern lights; in the Southern Hemisphere it is called the aurora australis. Auroras are the most visible effect of the sun's activity on the earth's atmosphere. The beautiful and often...

Accretion Disk
2004-10-19 04:45:41

Accretion Disk -- An accretion disk is a structure formed by material falling into a gravitational source. Conservation of angular momentum requires that, as a large cloud of material collapses inward, any small rotation it may have will increase. Centrifugal force causes the rotating cloud to collapse into a disk, and tidal effects will tend to align this disk's rotation with the rotation...