Tuesday, September 30, 2008

MOND vs Dark Matter


A couple of weeks ago, there was a Ph.D. thesis defence in Sharif University of Technology by Hossein Haghi **,***under supervision of Dr. Sohrab Rahvar/He is my supervisor too!:)/, in physics department with the title of MOND* in ... : Modified Newtonian Dynamics.
What is it about?and what is the story of modification...
During last century, in physics history usually we observed new phenomenas in experiments which could not be described by approved and used models and physical laws of the day.So what happened? Immediately the situation is divided in two ways
1)In the same frame of our known laws and models, we defined new physical entities to describe the experiment. or
2)We concluded that we do not know the laws well, and we start to change and modify them.

This scenario was happened several times, as a famous example is the problem of missing mass in beta decay in late 20's and beginning of 30's ,as I mentioned there were two solution 1)The law of conservation of energy is not correct or
2)there is a new mysterious unseen particle that we do not know about it and can not detect in experiment.

The history in this case shows that the second solution, the famous Neutrino predicted by Pauli was the right answer.
Now we have such a situation in galactic scale cosmology too.
The rotation curves of stars rotating in galaxy contradict with the prediction of Newtonian Mechanic, and we get flat rotational curves.

Immediately there is two solutions to the problem
1)Modifying the Newtonian law:Which means we do not understand it well,or there is more complete theory standing there.MOND as an example.
2)There is a new mysterious,only gravitationally interacting particle which named DARK MATTER ,which is responsible for this new missing mass in galaxy.
Finally what I want to mention is about the always existing two perspectives as a solution of the problem.
1)Modifying the laws or 2)Introducing New physical entity, are always teasing physicists minds.

But which one is seems more plausible is not yet well established but it will be fair to mention that the DARK MATTER campaign is so large /because of its success in cosmological model for structure formation models as CDM and there is a huge Hope to find light supersymmetric particles (LSP) in LHC arXiv:0807.2244 as a candidate for this entities as DARK MATTER.

But the story is not happy ended yet .Anyhow let us see what experiments unreveal for us.


*Nice web site about MOND related papers: http://www.astro.umd.edu/~ssm/mond/

Tuesday, September 23, 2008

Timeline of General Relativity and Cosmology from 1905


The new semester in Sharif university begins.This term Dr. Reza Mansouri,/ http://en.wikipedia.org/wiki/Reza_Mansouri /one of the prominent scientists of Iran teach General relativity and gravitation. Dr. Mansouri which recieved his Ph.D. in Austria in begining of 1970s is the first one who taught general relativity in Iran.
In first session he take a look on history of general relativity and cosmology.
The below is a more complete list of the happenings since the foundation of Special relativity in 1905.


1905 - Albert Einstein , special relativity

1907 - Albert Einstein introduces the principle of equivalence of gravitation and inertia

1915 - Albert Einstein - general relativity.

1915 - Karl Schwarzschild - Schwarzschild metric ,first solution to the Einstein field

1916 - Albert Einstein - field equations of general relativity admit wavelike solutions

1917 - Willem de Sitter derives an isotropic static cosmology with a cosmological constant, as well as an empty expanding cosmology with a cosmological constant, termed a de Sitter universe.

1918 - J. Lense and Hans Thirring find the gravitomagnetic precession of gyroscopes GR

1919 - Arthur Eddington -solar eclipse expedition- detection gravitational deflection of light

1921 - Theodor Kaluza 5-dimensional version of GR equations unifies gravitation and EM

1922 - Alexander Friedmann finds a solution to the Einstein field equations which suggests a general expansion of space

1927 - Georges Lemaître discusses the creation event of an expanding universe governed by the Einstein field equations.

1929 - Edwin Hubble demonstrates the linear redshift-distance relation and thus shows the expansion of the universe

1933 - Edward Milne names and formalizes the cosmological principle 1934 - Georges Lemaître interprets the cosmological constant as due to a vacuum energy with an unusual perfect fluid equation of state

1937 - Fritz Zwicky states that galaxies could act as gravitational lenses

1937 - Albert Einstein, Leopold Infeld, and Banesh Hoffmann -geodesic equations of GR can be deduced from its field equations

1938 - Paul Dirac suggests the large numbers hypothesis, that the gravitational constant may be small because it is decreasing slowly with time.

1948 - Hermann Bondi, Thomas Gold, and Fred Hoyle propose steady state cosmologies based on the perfect cosmological principle.

1948 - George Gamow predicts the existence of the cosmic microwave background radiation by considering the behavior of primordial radiation in an expanding universe

1950 - Fred Hoyle derisively coins the term "Big Bang".

1957 - John Wheeler discusses the breakdown of classical general relativity near singularities and the need for quantum gravity.

1960 - Robert Pound and Glen Rebka test the gravitational redshift predicted by EP the approximately 1%

1961 - Robert Dicke argues that carbon-based life can only arise when the gravitational force is small, because this is when burning stars exist; first use of the weak anthropic principle

1962 - Robert Dicke, Peter Roll, and R. Krotkov use a torsion fiber balance to test the weak equivalence principle to 2 parts in 100 billion

1964 - Irwin Shapiro predicts a gravitational time delay of radiation travel as a test GR

1965 - Joseph Weber puts the first Weber bar gravitational wave detector into operation

1965 - Martin Rees and Dennis Sciama analyze quasar source count data and discover that the quasar density increases with redshift.

1965 - Arno Penzias and Robert Wilson, astronomers at Bell Labs discover the 2.7 K microwave background radiation, which earns them the 1978 Nobel Prize in Physics. Robert Dicke, James Peebles, Peter Roll and David Todd Wilkinson interpret it as relic from the big bang.

1966 - Stephen Hawking and George Ellis show that any plausible general relativistic cosmology is singular

1966 - James Peebles shows that the hot Big Bang predicts the correct helium abundance

1967 - Andrei Sakharov presents the requirements for baryogenesis, a baryon-antibaryon asymmetry in the universe

1968 - Irwin Shapiro presents the first detection of the Shapiro delay

1968 - Kenneth Nordtvedt studies a possible violation of the weak equivalence principle for self-gravitating bodies and proposes a new test of the weak equivalence principle based on observing the relative motion of the Earth and Moon in the Sun's gravitational field.

1969 - Charles Misner formally presents the Big Bang horizon problem 1969 - Robert Dicke formally presents the Big Bang flatness problem.

1974 - Robert Wagoner, William Fowler, and Fred Hoyle show that the hot Big Bang predicts the correct deuterium and lithium abundances

1976 - Robert Vessot and Martin Levine use a hydrogen maser clock on a Scout D rocket to test the gravitational redshift predicted by the equivalence principle to approximately 0.007%

1976 - Gravity Probe A experiment confirmed slowing the flow of time caused by gravity matching the predicted effects to an accuracy of about 70 parts per million.

1979 - Dennis Walsh, Robert Carswell, and Ray Weymann discover the gravitationally lensed quasar Q0957+561

1981 - Viacheslav Mukhanov and G. Chibisov propose that quantum fluctuations could lead to large scale structure in an inflationary universe 1981 - Alan Guth proposes the inflationary Big Bang universe as a possible solution to the horizon and flatness problems

1982 - Joseph Taylor and Joel Weisberg show that the rate of energy loss from the binary pulsar PSR B1913+16 agrees with that predicted by the general relativistic quadrupole formula to within 5%

1990 - Preliminary results from NASA's COBE mission confirm the cosmic microwave background radiation is an isotropic blackbody to an astonishing one part in 105 precision, thus eliminating the possibility of an integrated starlight model proposed for the background by steady state enthusiasts.

1990s - Ground based cosmic microwave background experiments measure the first peak, determine that the universe is geometrically flat

1998 - Controversial evidence for the fine structure constant varying over the lifetime of the universe is first published.

1998 - Adam Riess, Saul Perlmutter and others discover the cosmic acceleration in observations of Type Ia supernovae providing the first evidence for a non-zero cosmological constant.

1999 - Measurements of the cosmic microwave background radiation (most notably by the BOOMERanG experiment see Mauskopf et al., 1999, Melchiorri et al., 1999, de Bernardis et al. 2000) provide evidence for oscillations (peaks) in the anisotropy angular spectrum as expected in the standard model of cosmological structure formation. These results indicates that the geometry of the universe is flat. Together with large scale structure data, this provides complementary evidence for a non-zero cosmological constant.

2002 - The Cosmic Background Imager (CBI) in Chile obtained images of the cosmic microwave background radiation with the highest angular resolution of 4 arcmin. It also obtained the anisotropy spectrum at high-resolution not covered before up to l ~ 3000. It found a slight excess in power at high-resolution (l > 2500) not yet completely explained, the so-called "CBI-excess".

2003 - NASA's WMAP obtained full-sky detailed pictures of the cosmic microwave background radiation. The image can be interpreted to indicate that the universe is 13.7 billion years old (within one percent error) and confirm that the Lambda-CDM model and the inflationary theory are correct. 2003 - The Sloan Great Wall is discovered.

2004 - The Cosmic Background Imager first obtained the E-mode polarization spectrum of the cosmic microwave background radiation.

2006 - The long-awaited three-year WMAP results are released, confirming previous analysis, correcting several points, and including polarization data.

2007 - End of Gravity Probe B experiment.

Monday, September 15, 2008

Primordial non-gaussianity in cosmology




Couple of days ago Niayesh Afshordi from Perimeter Institute visited to Sharif university of technology, physics department.He had a talk about primordial non Gausianity in cosmology.It seems that will be interesting and hot topic of research in upcoming decade, because of new and exact experiments that we will have.
This is the title and abstract of his talk.
Title: Primordial non-gaussianity in cosmology
Abstract: "Modern cosmological observations seem to indicate that theinitial conditions of the universe obey gaussian statistics withremarkable accuracy, as predicted by simple inflationary models.Nevertheless, any possible signature of primordial non-gaussianity will provide a unique window into physics of the very early universe. In this talk, I will outline the various ways through which these signatures could be observed, and what they may imply for the early universe physics. Inparticular, I will talk about recent developments that show primordialnon-gaussianity will affect large scale structure of galaxy distribution significatntly, and already provides comparable constraints to the cosmicmicrowavebackground anisotropies."








Article of Niayesh about primordial non gusianity:




Sunday, September 14, 2008

GRB in z=6.7


Gamma ray bursts (GRB)/are the most luminous electromagnetic events occurring in the universe since the Big Bang. They are flashes of gamma rays emanating from seemingly random places in deep space at random times/, are lately in focus of great attention.The speculation that maybe they would be possible standard candles make their importance more.

If there are standard candles then we can track the expansion history of universe in higher redshifts,cause they founded farther than supernovas.The farthest Gamma ray burst is founded in z=6.7 the report of this GCN is reported in work of Fynbo et al.This is the report.


"
TITLE: GCN CIRCULAR NUMBER: 8225
SUBJECT: GRB 080913:
VLT/FORS spectrum
DATE: 08/09/13 17:28:26 GMT
J.P.U. Fynbo (DARK/NBI), J. Greiner, T. Kruehler (both MPE),A. Rossi (Tautenburg Obs), P. Vreeswijk, D. Malesani (both DARK/NBI)report on behalf of a larger collaboration
Based on the rapid determination of an "i-band" drop out we triggered VLT/ToO observations (program-ID 081.A-0135; PI Greiner) and obtained spectroscopy with FORS2+Gris600z of the afterglow of GRB080913(Schady et al. 2008, GCN #8217; Stamatikos et al. 2008, GCN #8222*.
Observations started at 8:30, about 100 min after the GRB, and continueduntil morning twilight. We detect a red continuum that disappears bluewards of a break around 9400 AA. Interpreting this break as the onset of the Lyman-alpha forest we infer a redshift of about z=6.7 consistent with the photometric redshift determined by GROND (Greiner et al., GCN #8223)."

But the physics of GRB is undiscovered yet.,further investigations may assure us about their physics.
footnote:
Thanks to Amir a Gamma ray burst investigator in Michigan tech physics department.

Thursday, September 11, 2008

M. Dominik In Tehran & Microlensing method for exoplant finding


One of the hottest topics in Astronomy is EXOPLANETS founding.
Finding exoplanets are the first steps in order to find life in our universe!!!
There are many methods in order to find the exoplanets,like astometry, spectroscopy, straight observation and...
A new physical method is gravitational microlensing mainly become in attention after founding of OGLE group by Alcock et al . published in Nature which they found and exoplanet by this method for the first time.
The story is the old one of Einstein's bending light story.When a light ray pass by from a massive object /in this case the gravitational lens a star a galaxy or cluster of galaxies/ light beams bend, consequently the shape and flux magnitude of images of the source are altered,this phenomena is named as gravitational lensing.
In 1919 Eddington's observation of gravitational lensing is the first stone like prove of general relativity view of nature.
But what is gravitational microlensing. When the lens is a star which magnify the energy flux of a distant source caused to microlensing effect.The naming comes after the situation that the effect of light bending produce new images of source which are separated in sky by micro seconds angle,which with our nowadays technology is hard to detect. So the only effect we will see is the magnification of light.
Now this method is used to finding exoplanets. If the lens is consisting of a star and planet the magnification curve which is called after Paszynski is lost its symmetry and a bump appear in it
which is a first clue of an binary lens, maybe a exoplanet!!!
This method is fine in order to find planets of less mass which will be suitable for finding life there. Anyhow this method needs tremendous effort in computer work, curve analysis and data management.
But the news is that Martin Dominik professor of physics from Saint Andrews which is a really expert in gravitational microlensing and exoplant finding/see the second link/ visiting Tehran, and has nice couple of lectures in IPM in 10-11 September.

Wednesday, September 10, 2008

LHC Starts!!!


Finally after a two decade scientific planning and work, today the Cern Large Hadron Collider /near Geneva in the border of France and Switzerland/starts it works.
This tremendous,huge scientific projects, I think, must be included in the new eras one of world 7 wonders list.This super collider in energy and dimensions /27 km in circumstance tunnel /is the biggest one ever build, reaching to energy scales of TeV,which will be suitable for physicist to trace the physics beyond standard model.
There is a great hope to find the missing golden part of standard model the higgs boson.
Although there is chances in finding light supersymmetric particles/LSP/ which theoretically are presented as Dark Matter candidates.

The opening experiment excitement of LHC is reflected well in cosmic variance live blog.
Even today the Google changes its logo in honor of this human achievement.
Now there will be opened a golden era for particle physics phenomenologists to grasp the next decades Nobel prizes.

Monday, September 8, 2008

Cosmology a monumental work by Weinberg



Steven Weinberg is really a great TITAN standing in the edge of new physics.
In April 2008, Steven's new book in cosmology which named briefly COSMOLOGY is printed by Oxford university press.
What a huge ,tremendous work,like his last decade Quantum field theory books ,this one is an encyclopedia too.
Weinberg claims that he tried to cover the most advanced research areas of cosmology up to 2007 also in the introduction he mentioned that after 1999 as he finished the QFT 3 volume work he had a plenty of time studying cosmology,he read the review articles but not satisfied with them,He drives all the cosmology by himself!!!

It starts from Accelerating universe to Gravitational microlensing,what a work!
Andrei Linde from Stanford declare:
"A monumental book written by a leading authority in particle physics and cosmology. Since publication of Weinberg's famous book Gravitation and Cosmology 35 years ago, there has been a real revolution both in cosmological theory and observations. A major effort of a great expert has been required to summarize the main developments in one book, and to make this presentation both highly accurate and accessible. This book will be greatly appreciated by a broad readership, ranging from students who just enter the field to experts in modern cosmology. It should be on the desk of every actively working cosmologist."

Robert Kirshner from Harvard
"This book tackles the main events of today's cosmology: cosmic acceleration observed with supernovae, the exquisite structure of the cosmic microwave background, and the evidence for dark matter. Weinberg pays close attention to the historical development and summarizes the observations with care. He brings deep knowledge of the underlying physics and weaves these threads together into a rich text that will be of great value to astronomers and physicists. The first half of this book is a wonderful introduction to cosmology, suitable for a graduate course or for someone coming into the field from a neighboring region of the scientific forest. The second half is an original development of the theory for the growth of inhomogeneities in the Universe. Everyone who works on cosmology will find something to learn in this book."


http://en.wikipedia.org/wiki/Steven_Weinberg

Product Details
Hardcover: 544 pages
Publisher: Oxford University Press, USA (April 28, 2008)
Language: English
ISBN-10: 0198526822
ISBN-13: 978-0198526827
Product Dimensions: 10 x 7 x 1.5 inches
Anyhow today I start my journey through Weinbergs Cosmology!!!