HST and 'dark matter' |
HST and 'dark matter' |
Guest_PhilCo126_* |
May 11 2007, 05:13 PM
Post
#1
|
Guests |
ASA Updates Plans for Hubble 'Ring Of Dark Matter' Briefing
GREENBELT, Md. - NASA will hold a media teleconference at 1 p.m. EDT on May 15 to discuss the strongest evidence to date that dark matter exists. This evidence was found in a ghostly ring of dark matter in the cluster CL0024+17, discovered using NASA's Hubble Space Telescope. The ring is the first detection of dark matter with a unique structure different from the distribution of both the galaxies and the hot gas in the cluster. The discovery will be featured in the June 20 issue of the Astrophysical Journal. |
|
|
Mar 11 2010, 05:46 AM
Post
#2
|
|
Member Group: Members Posts: 723 Joined: 13-June 04 Member No.: 82 |
Part 2 of 2
Modified gravity emerging from thermodynamics and holographic principle -- Yi Zhang, Yun-gui Gong, Zong-Hong Zhu QUOTE We discuss three different corrections to the area law of entropy. The number of bits N is then modified according to the form of entropy, and the law of gravity is obtained by using the method developed by Verlinde. We also discuss the effect of the entropy correction on the Friedmann equation, and we find that the Friedmann equation got an extra term H^n, which can be used to explain the current accelerating expansion of the universe. The result suggests that dark energy may be an emergent phenomenon based on the holographic principle and thermodynamics. Towards a Holographic Description of Inflation and Generation of Fluctuations from Thermodynamics -- Yi Wang QUOTE Recently, Verlinde conjectured that gravity may be an entropic force, arising from thermodynamics on the holographic screen. We investigate the implications of the entropic force formalism for inflationary cosmology. We find the background dynamics of inflation can be dually described in the holographic language. At the perturbation level, two kinds of novel scale invariant scalar fluctuations arise from thermal fluctuations on the holographic screen. These fluctuations can be responsible for CMB anisotropy and structure formation. Is Holographic Entropy and Gravity the result of Quantum Mechanics? -- Joakim Munkhammar QUOTE In a remarkable paper Verlinde recently proposed a framework for gravity as an entropic force. This theory, while related to Jacobsson's approach, showed that Newtonian gravity easily could be obtained by using entropic and holographic arguments. The assumption was that space is emergent and that the holographic principle holds. Bekenstein entropy was also a key component in his approach. He thus reversed the line of research, assuming that the holographic principle was underlying Newtonian physics. The change of entropy was linked to the change of the Newtonian potential, this led to the conclusion that inertia might be equivalent to the lack of entropy gradients. As Verlinde states, the holographic principle has not been easy to extract from the laws of Newton and Einstein because it is deeply hidden within them. His paper attracted quite some attention and several papers from various fields of theoretical physics, including Loop Quantum Gravity, have been published relating to its topic. A shortcoming of the theory was the unknown origin of the coupling constant h~. This coupling constant was added by Bekenstein in the 1970s mainly for dimensional reasons and has since remained a mystery. We will suggest an origin of this constant in this paper.
In Verlinde's view space is mainly a storage place of information, which is associated with positions, movements and mass of matter. This information is displayed to us on a surface, a screen in front of us. The information is stored in discrete bits on the screen and since the number of bits is limited we get holographic effects. This means that if there is more information on the inside than the amount of information accessible on the screen then information will be hidden from us as we observe the dynamics. This is the holographic principle. Thus the dynamics on the screen is governed by some unknown rules which then only can utilize the information on the screen. Since information is stored on a screen this means that space is emergent in the normal direction of the screen. The microstates may be thought of having all sorts of physical attributes such as energy, temperature etc. This is then related, via entropy, to the information associated with the system. [...] The great connections between matter and information made by Bekenstein, Hawking and others in the 1970s has turned out to have very interesting consequences. Verlinde's framework for the origin of the laws of Newton including gravitation based on entropy is perhaps one of the greatest consequences of this. In what way space is emergent and how the holographic principle holds is starting to fall into place. In this paper we have proposed an entropy arising from quantum mechanics and we have investigated its relation to Verlinde's theory. This was then applied to generalize the Newtonian potential arising in Verlinde's theory. There are many open problems remaining as this is a theory in progress. The study of multiple particle situations in the quantum mechanical approach should be interesting. A relativistic approach to quantum entropic gravity also needs to be established and investigated. The construction of various quantum field theories in curved spacetimes based on this approach should be of particular interest. Generally the connection to quantized gravity theories, if there are valid such, is left for further investigation. The nature of space and time as derived concepts, as spoken of by Verlinde, is not addressed in this paper and are features in need of investigation. The relation to AdS/CFT correspondence is also left open for further investigation. In conclusion, this paper, guided by a pure speculation, suggests that the gravitational attraction perhaps could be the result of a particular type of entropy arising in quantum mechanics. Verlinde proposed a theory of gravity where gravity no longer was a fundamental force, but rather an effect of entropy. His theory, as a reversal of physics research, is a remarkable framework that has many open questions and interesting consequences still to be uncovered. This is why the greatest achievement of this paper is that it provides Verlinde's remarkable theory with a possible physical explanation for the factor h~, which had previously been added by Bekenstein mainly for dimensional reasons. |
|
|
Lo-Fi Version | Time is now: 31st October 2024 - 10:59 PM |
RULES AND GUIDELINES Please read the Forum Rules and Guidelines before posting. IMAGE COPYRIGHT |
OPINIONS AND MODERATION Opinions expressed on UnmannedSpaceflight.com are those of the individual posters and do not necessarily reflect the opinions of UnmannedSpaceflight.com or The Planetary Society. The all-volunteer UnmannedSpaceflight.com moderation team is wholly independent of The Planetary Society. The Planetary Society has no influence over decisions made by the UnmannedSpaceflight.com moderators. |
SUPPORT THE FORUM Unmannedspaceflight.com is funded by the Planetary Society. Please consider supporting our work and many other projects by donating to the Society or becoming a member. |