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Topic: Particle horizon

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	NationMaster - Encyclopedia: Particle horizon
		The particle horizon in physical cosmology is the maximum distance from which particles (of positive or zero mass) can have travelled to the observer in the age of the universe.
		If a particle is moving at a constant velocity in a non-expanding universe free of gravitational fields, any event that occurs in that universe will eventually be observable by the particle, because the forward light cones from these events intersect the particle's world line.
		For the case of a horizon perceived by an occupant of a de Sitter universe, the horizon always appears to be a fixed distance away for a non-accelerating observer.
	www.nationmaster.com /encyclopedia/Particle-horizon (702 words)

	Particle Blogger Demo :: Homepage
		There is also another revision release of Particle Blogger on the horizon as we're working to add some extra stuff to it at the moment, most notably automatic Digg links so that your users can quickly and easily Digg your blog.
		You may have noticed that we recently released a new version of Particle Whois, the main focus of which was a vastly improved admin interface.
		The reason for the delay on the new version of Particle Blogger is that we are still tackling the new and upgraded WYSIWYG editor for writing and editing posts.
	blogger.particlesoft.net (1839 words)

	Event horizon Summary
		If a particle is moving at a constant velocity in a non-expanding universe free of gravitational fields, any event that occurs in that universe will eventually be observable by the particle, because the forward light cones from these events intersect the particle's world line.
		For the case of a horizon perceived by an occupant of a de Sitter universe, the horizon always appears to be a fixed distance away for a non-accelerating observer.
		For accelerating particles, this manifests as the Unruh effect, which causes space around the particle to appear to be filled with matter and radiation.
	www.bookrags.com /Event_horizon (1932 words)

	Event horizon - Wikipedia, the free encyclopedia
		If a particle is moving at a constant velocity in a non-expanding universe free of gravitational fields, any event that occurs in that universe will eventually be observable by the particle, because the forward light cones from these events intersect the particle's world line.
		For the case of a horizon perceived by an occupant of a de Sitter universe, the horizon always appears to be a fixed distance away for a non-accelerating observer.
		For accelerating particles, this manifests as the Unruh effect, which causes space around the particle to appear to be filled with matter and radiation.
	en.wikipedia.org /wiki/Event_horizon (1719 words)

	Particle horizon - Wikipedia, the free encyclopedia
		The particle horizon in physical cosmology is the maximum distance from which particles (of positive or zero mass) can have travelled to the observer in the age of the universe.
		The particle horizon is defined as the largest comoving distance from which light can have reached the observer — at the present moment.
		The event horizon is defined as the largest comoving distance from which light can ever reach the observer — at any time in the future.
	en.wikipedia.org /wiki/Particle_horizon (124 words)

	BBC - Science & Nature - Horizon
		Horizon producer David Sington on why predictions about the Earth's climate will need to be re-examined.
		Because the particles seed the formation of water droplets, polluted clouds contain a larger number of droplets than unpolluted clouds.
		As things stand, CO2 levels are projected to rise strongly over coming decades, whereas there are encouraging signs that particle pollution is at last being brought under control.
	www.bbc.co.uk /sn/tvradio/programmes/horizon/dimming_prog_summary.shtml (756 words)

	JamBase \| PARTICLE SPRING TOUR
		Particle is poised to return to the stage February 24 at The Fonda in Hollywood with the help of a few friends.
		Particle's return to the Midwest is complete with confirmed appearances in Lawrence at the Granada, Minneapolis at The Cabooze, Madison at The Orpheum Stage Door, Chicago at The Abbey Pub, and rounding out with Detroit at The Magic Stick.
		Particle is going the distance this spring hitting east coast cities in mid April beginning with a special 4/20 show at The Paradise in Boston with Pnuma Trio.
	www.jambase.com /headsup.asp?storyID=7901 (702 words)

	Horizon Laminates Ltd. - Laminate Kitchen Counter Tops
		Particle Board specifications – at present Horizon uses 5/8” particle board in 30” x 96”, 30” x 120”, 30” x 144”.
		With the automatic spray system, the sheet of laminate is sprayed with a neoprene contact adhesive and then is followed immediately by the particle board core.
		The shipping department at Horizon Laminates uses experience and professional judgement to determine the size of a package and when a pallet is to be used.
	www.horizonlaminates.com /page_productionPlant.htm (468 words)

	Particle horizon - Definition, explanation
		The particle horizon in cosmology is the distance from which particles (of positive mass or of zero mass) can have travelled to the observer in the age of the Universe.
		The particle horizon is defined as the largest comoving distance from which light can have reached us (the observer) - today.
		The event horizon is defined as the largest comoving distance from which light will ever reach us (the observer) - at any time in the future.
	www.calsky.com /lexikon/en/txt/p/pa/particle_horizon.php (146 words)

	The Universe Around Us: Chapter 6 (Site not responding. Last check: )
		This feature implies that we can only see out to those particles whose present-day distance corresponds to the age of the Universe; the particles beyond cannot be seen by us no matter what detectors we may use (light has not had time to travel to us from them since the creation of the Universe).
		Actually we cannot even see as far as the particle horizon, because the Universe is opaque at early times (before decoupling), as explained in the previous chapter.
		Corresponding to the particle horizon, there is a physics horizons limiting our ability to confirm the nature of physics controlling events in the very earliest stages of the expansion of the universe.
	www.mth.uct.ac.za /~ellis/cos6.html (6042 words)

	Before the Big Bang?
		If we follow this model backwards in time to when the Universe was very hot and dense, and dominated by radiation, then we have to understand the particle physics that happens at such high densities of energy.
		Particle collisions cannot move information faster than the speed of light.
		But in the expanding Universe that we appear to live in, photons moving at the speed of light cannot get from one side of the Universe to the other in time to account for this observed isotropy in the thermal radiation.
	www.superstringtheory.com /cosmo/cosmo4.html (728 words)

	Before the Big Bang?
		The experimental understanding of particle physics starts to poop out after the energy scale of electroweak unification, and theoretical physicists have to reach for models of particle physics beyond the Standard Model, to Grand Unified Theories, supersymmetry, string theory and quantum cosmology.
		The horizon size of our Universe today is too small for the isotropy in the cosmic microwave background to have evolved naturally by thermalization.
		Particle theories like Grand Unified Theories and superstring theory predict magnetic monopoles should exist, and relativity tells us that the Big Bang should have produced a lot of them, enough to make one hundred billion times the observed energy density of our Universe.
	superstringtheory.com /cosmo/cosmo4.html (728 words)

	What is a Particle Counter?
		Airborne particle counters are sophisticated instruments that measure airborne particles in a number of size ranges which are far too small to be seen, but may have a serious impact on air quality, and therefore personal health and safety.
		The counted particles are trapped in an exhaust filter and not returned to the environment from which the sample was taken.
		They report that the military now employs portable particle counters to detect particulate level ‘events’ or surges that indicate the presence of new contaminants, and integrates these counters with test stations, which submit the sample to immediate testing to determine if the particles are biologically viable, and whether they match known threat profiles.
	www.particle.com /whitepapers_met/iaq.htm (1725 words)

	Myswizard » Event Horizon (Site not responding. Last check: )
		An event horizon is a boundary in spacetime for a given observer beyond which no electromagnetic energy, including light, can reach the observer.
		It is beyond the event horizon for that particle.
		The world line of the observer is given as the solid curve in a two dimensional spacetime representation with time x0 in the vertical direction and a one dimensional space coordinate x3 to the right.
	www.myswizard.com /2005/12/30/event-horizon (916 words)

	27
		Yet cosmology disallows the presence of these monopoles: their energy density would drive the expansion of the universe too fast, so that a universe with the parameters we observe today would be too young (that is, observed objects would be older than the inferred age of the universe).
		Particle physics' prediction of relic monopoles whose high density was ruled out by cosmology was called the ``monopole problem''.
		We described reheating as the transfer of that former bulk potential energy into particle creation, populating and reheating the universe to nonzero temperature and creating lots of entropy, from which point the universe is RD and evolves as in the standard thermal history.
	www.emory.edu /PHYSICS/Faculty/Benson/380-04/notes/27/27.html (965 words)

	Orbits in Strongly Curved Spacetime
		Kepler's laws of planetary motion, grounded in Newton's theory of gravity, state that the orbit of a test particle around a massive object is an ellipse with one focus at the centre of the massive object.
		With sufficient angular momentum, a particle can approach the event horizon as closely as it wishes (assuming it is small enough so it isn't torn apart by tidal forces), but it can never cross the event horizon and return.
		Proper time on the particle continues to advance unabated; an observer on-board sails through the event horizon without a bump and continues toward the doom which awaits at the central singularity.
	www.fourmilab.ch /gravitation/orbits (2354 words)

	Cosmological Horizons
		A type of horizon which is fairly certain to exist is what is known as a particle horizon.
		Note that graphs showing particle horizons are generally confusing, as they often show a background of the other parts of the universe evolving with time, implying that at a certain stage of their evolution they cross the particle horizon.
		The other thing to note is that for particle horizons to occur the rate of expansion of the universe (the derivative of the scale factor with time) must be infinite at time zero.
	www.chronon.org /articles/cosmichorzns.html (944 words)

	Hawking Radiation as Delayed Choice
		The event horizon is the surface at which the forward light cones are tilted inward to the extent that they are entirely contained inside the surface.
		For any realistic particle, there must have been a time prior to when the particle began its uniform acceleration, and this change in acceleration results in radiation, in a sense because the Fourier transform of the particle’s motion acquires impurities from the change in acceleration.
		The red curve signifies the worldline of an in-falling particle, and the gold curve represents a lightlike (null) locus from the in-falling particle to the exterior region in the future.
	www.mathpages.com /home/kmath591/kmath591.htm (1246 words)

	Lecture 43: The Very Early Universe
		Because the universe became transparent early in its history, long before the present day, the distance to the opaque ``fog bank'' is nearly equal to the our cosmic particle horizon distance.
		Points A and B are separated by a distance larger than the horizon distance.
		Before the year 1980, cosmologists were baffled by the horizon problem and the flatness problem.
	www-astronomy.mps.ohio-state.edu /~ryden/ast162_10/notes43.html (1234 words)

	Cosmic Particle Horizon
		I understand that the cosmic particle horizon is located about 20 billion light years away, in all directions, from me. Light coming from objects located past the horizon cannot be seen because the light has not had enough time to reach me yet.
		But doesn't the cosmic particle horizon expand with the expansion of the Universe.
		The cosmic particle horizon is related to the age of the Universe.
	imagine.gsfc.nasa.gov /docs/ask_astro/answers/970328c.html (220 words)

	What about string theory?
		In cosmology, the event horizon is like the particle horizon, except that it is in the future and not in the past.
		But a cosmological event horizon is a major technical problem in high energy physics, because of the definition of relativistic quantum theory in terms of the collection of scattering amplitudes called the S Matrix.
		One of the fundamental assumptions of quantum relativistic theories of particles and strings is that when incoming and outgoing states are infinitely separated in time, they behave as free noninteracting states.
	superstringtheory.com /cosmo/cosmo5.html (1251 words)

	Particle and event horizons
		The particle horizon is related to the past and the event horizon is related to the future.
		The particle horizon grows always and the event horizon tends to the Hubble sphere (c / H) in case of a cosmological model dominated by the cosmological constant (like the current one).
		Objets beyond the event horizon but inside the particle horizon may be for example very far quasars; their light, emitted in past, is reaching us now, but the light they are supposedly emitting now, will never reach us in future.
	www.physicsforums.com /showthread.php?t=74429 (269 words)

	Forums - Is the speed of light really constant ?
		But a cosmological event horizon is a major technical problem in high energy physics, because of the definition of relativistic quantum theory in terms of the collection of scattering amplitudes called the S Matrix.
		One of the fundamental assumptions of quantum relativistic theories of particles and strings is that when incoming and outgoing states are infinitely separated in time, they behave as free noninteracting states.
		But the presence of an event horizon implies a finite Hawking temperature and the conditions for defining the S Matrix cannot be fulfilled.
	www.elitetrader.com /vb/showthread.php?threadid=28193 (847 words)

	Spartanburg SC \| GoUpstate.com \| Spartanburg Herald-Journal
		It represents the portion of the universe which we could have conceivably observed by the present day.
		In terms of comoving distance the particle horizon is equal to the conformal time
		The particle horizon differs from the event horizon in that the particle horizon represents the largest comoving distance from which light could have reached us by now, while the event horizon is the largest comoving distance from which light emitted now can ever reach the observer at any time in the future.
	www.goupstate.com /apps/pbcs.dll/section?category=NEWS&template=wiki&text=Particle_horizon (174 words)

	Big Bang Theory - Origin of Universe - 0rig.in (Site not responding. Last check: )
		This component of the Universe's composition is revealed by its property of causing the expansion of the Universe to deviate from a linear velocity-distance relationship by causing spacetime to expand faster than expected at very large distances.
		The co-moving distances and conformal times are defined so that objects moving with the cosmological flow are always the same co-moving distance apart and the particle horizon or observational limit of the local Universe is set by the conformal time.
		The horizon problem results from the premise that information cannot travel faster than light, and hence two regions of space which are separated by a greater distance than the speed of light multiplied by the age of the Universe cannot be in causal contact.
	www.0rig.in /cosmology/big_bang_theory.htm (4780 words)

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