Dark Energy and Dark Matter:

The Results of Flawed Physics?

http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/

 

 

Dark Energy, Dark Matter 

 

 

In the early 1990's, one thing was fairly certain about the expansion of the Universe. It might have enough energy density to stop its expansion and recollapse, it might have so little energy density that it would never stop expanding, but gravity was certain to slow the expansion as time went on. Granted, the slowing had not been observed, but, theoretically, the Universe had to slow. The Universe is full of matter and the attractive force of gravity pulls all matter together. Then came 1998 and the Hubble Space Telescope (HST) observations of very distant supernovae that showed that, a long time ago, the Universe was actually expanding more slowly than it is today. So the expansion of the Universe has not been slowing due to gravity, as everyone thought, it has been accelerating. No one expected this, no one knew how to explain it. But something was causing it.

Eventually theorists came up with three sorts of explanations. Maybe it was a result of a long-discarded version of Einstein's theory of gravity, one that contained what was called a "cosmological constant." Maybe there was some strange kind of energy-fluid that filled space. Maybe there is something wrong with Einstein's theory of gravity and a new theory could include some kind of field that creates this cosmic acceleration. Theorists still don't know what the correct explanation is, but they have given the solution a name. It is called dark energy.

Author by: 

(Robert Roy Britt)

Edited By:

Arip Nurahman

Dark matter and dark energy are two of the most vexing problems in science today. Together they dominate the universe, comprising some 96 percent of all mass and energy.

But nobody knows what either is. It's tempting to consider them products of the same unknown phenomenon, something theorist Robert Scherrer suggests. The professor of physics at Vanderbilt University says "k-essence" is behind it all.

Dark matter was invoked decades ago to explain why galaxies hold together. Given regular matter alone, galaxies might never have formed, and today they would fly apart. So there must be some unknown stuff that forms invisible clumps to act as gravitational glue.

Dark energy hit the scene in the late 1990s when astronomers discovered the universe is not just expanding, but racing out at an ever-faster pace. Some hidden force, a sort of anti-gravity, must be pushing galaxies apart from one another in this accelerated expansion.

Separate theories have been devised to try and solve each mystery.

To explain dark energy, for example, theorists have re-employed a "cosmological constant" that Einstein first introduced as a fudge factor to balance the force of gravity. Einstein called the cosmological constant a great blunder and retracted it. Yet many theorists now are comfortable re-employing it to account for the effects of dark energy. But it does not reveal what the force is.

Scherrer agrees two explanations might be necessary, but he's also bothered by that complexity.

Best fit theoretical rotation curves superimposed on data (dotted lines) from galaxy “NGC 4455” (left) and galaxy “NGC 5023” (right). The solid line is the curve predicted by the new gravity model. Also shown are the Newtonian curve (short dashes) and the Newtonian curve corrected for dark matter (long dashes). There are few scientific concepts as intriguing and mysterious as dark energy and dark matter, said to make up as much as 95 percent of all the energy and matter in the universe. And even though scientists don't know what either is and have little evidence to prove they exist, dark energy and dark matter are two of the biggest research problems in physics.

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This is what three Italian physicists have recently asked. In a paper in the August 3 online edition of the Institute of Physics' peer-reviewed Journal of Cosmology and Astroparticle Physics, they put forth the idea that scientists were forced to propose the existence of dark energy and dark matter because they were, and still are, working with incorrect gravitational theory.

The group suggests an alternative theory of gravity in which dark energy and dark matter are effects – illusions, in a sense – created by the curvature of spacetime (the bending of space and time caused by extremely massive objects, like galaxies). Their theory does not require the existence of dark energy and dark matter.

“Our proposal implies that the 'correct' theory of gravity may be one based solely on directly observed astronomical data,” said lead author Salvatore Capozziello, a theoretical physicist at the University of Naples, to PhysOrg.com.

Dark energy and dark matter were originally conceived to explain, respectively, the accelerating expansion of the universe (despite the tendency of gravity to push matter together) and the discrepancy between the amount of matter scientists expect to observe in the universe but have not yet found. Astronomers suggested the existence of dark matter when they noticed something odd about spiral galaxies: Stars at the middle and edge of a spiral galaxy rotate just as fast as stars near the very center. But according to Newtonian mechanics (the physics of bodies in motion), stars further away from the galactic center should rotate more slowly. Scientists thus assumed that some sort of “dark” matter, not observable by emitted light, must be boosting the total gravity of the galaxy, giving the stars extra rotational speed.

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“We can show that no 'exotic' ingredients have to be added to fill the gap between theory and observations,” said Capozziello.

In their paper, he and his co-authors demonstrate this using data from 15 well-studied galaxies. Among this data was each galaxy's “rotation curve,” a graph that plots the rotational speed of the stars in the galaxy as a function of their distance from the galaxy's center. These curves were successfully fit to curves produced using the new theory. Since these 15 galaxies are believed to be dominated by dark matter, fitting their rotation curves using this new gravity model is strong evidence to support an alternative theory of gravity.

Despite this, the notion that dark matter and dark energy are “wrong” is potentially very unpopular. Capozziello and his colleagues are aware that a new theory of gravity impacts the dynamics of the universe as scientists now understand them.

“Any extended theories of gravity must be tested on all the astrophysical scales, ranging from the Solar System to galaxies to galaxy clusters, and all of cosmology,” said Capozziello. “Performing these tests is the cornerstone of our research program.”

See also

• Cosmic inflation
• Dark flow
• De Sitter relativity
• Lambda-CDM model
• Vacuum energy
• Zero-point field

Bibliography

• HubbleSite press release: New Clues About the Nature of Dark Energy: Einstein May Have Been Right After All.
• 1998 paper announcing the dark energy discovery: Riess et al.
• 1999 paper confirming dark energy discovery Perlmutter et al..
• The group that first detected cosmic acceleration: High-Z supernova search team and the group that confirmed it Supernova Cosmology Project.
• Sean Carroll's technical reviews: Why is the universe accelerating?, The Cosmological Constant, and Dark Energy and the Preposterous Universe.
• Jim Peebles, Testing General Relativity on the Scales of Cosmology.
• "The World's Most Successful Nearby Supernova Search Engine", The Katzman Automatic Imaging Telescope.
• Supernova Acceleration Probe (SNAP), a proposed satellite experiment.
• A reanalysis^[38] of an experiment by Koch et al.^[39] to find the broad-band spectrum of Josephson junction noise current claims to connect it to the spectral frequency upper limit predicted by matching estimates of the dark energy density to the measured vacuum energy density. This claim is not yet accepted. For disputes, see.^[40][41][42]
• Christopher J. Conselice, "The Universe's Invisible Hand," Scientific American. February, 2007.

References

1. ^ P. J. E. Peebles and Bharat Ratra (2003). "The cosmological constant and dark energy". Reviews of Modern Physics 75 (2): 559–606. arXiv:astro-ph/0207347. Bibcode 2003RvMP...75..559P. doi:10.1103/RevModPhys.75.559. 
2. ^ ^{a b} "Seven-Year Wilson Microwave Anisotropy Probe (WMAP) Observations: Sky Maps, Systematic Errors, and Basic Results" (PDF). nasa.gov. http://lambda.gsfc.nasa.gov/product/map/dr4/pub_papers/sevenyear/basic_results/wmap_7yr_basic_results.pdf. Retrieved 2010-12-02.  (see p. 39 for a table of best estimates for various cosmological parameters)
3. ^ ^{a b} Sean Carroll (2001). "The cosmological constant". Living Reviews in Relativity 4. http://relativity.livingreviews.org/Articles/lrr-2001-1/index.html. Retrieved 2006-09-28. 
4. ^ L.Baum and P.H. Frampton (2007). "Turnaround in Cyclic Cosmology". Physical Review Letters 98 (7): 071301. arXiv:hep-th/0610213. Bibcode 2007PhRvL..98g1301B. doi:10.1103/PhysRevLett.98.071301. PMID 17359014. 
5. ^ ^{a b} Adam G. Riess et al. (Supernova Search Team) (1998). "Observational evidence from supernovae for an accelerating universe and a cosmological constant". Astronomical J. 116 (3): 1009–38. arXiv:astro-ph/9805201. Bibcode 1998AJ....116.1009R. doi:10.1086/300499. 
6. ^ ^{a b} S. Perlmutter et al. (The Supernova Cosmology Project) (1999). "Measurements of Omega and Lambda from 42 high redshift supernovae". Astrophysical J. 517 (2): 565–86. arXiv:astro-ph/9812133. Bibcode 1999ApJ...517..565P. doi:10.1086/307221. 
7. ^ The first paper, using observed data, which claimed a positive Lambda term was G. Paal et al. (1992). "Inflation and compactification from galaxy redshifts?". ApSS 191: 107–24. Bibcode 1992Ap&SS.191..107P. doi:10.1007/BF00644200. 
8. ^ "The Nobel Prize in Physics 2011". Nobel Foundation. http://nobelprize.org/nobel_prizes/physics/laureates/2011/index.html. Retrieved 2011-10-04. 
9. ^ The Nobel Prize in Physics 2011. Perlmutter got half the prize, and the other half was shared between Schmidt and Riess.
10. ^ ^{a b} D. N. Spergel et al. (WMAP collaboration) (March 2006). Wilkinson Microwave Anisotropy Probe (WMAP) three year results: implications for cosmology. http://lambda.gsfc.nasa.gov/product/map/current/map_bibliography.cfm.

Materi Gelap

Materi gelap adalah materi yang tidak dapat dideteksi dari radiasi yang dipancarkan atau penyerapan radiasi yang datang ke materi tersebut, tetapi kehadirannya dapat dibuktikan dari efek gravitasi materi-materi yang tampak seperti bintang dan galaksi. Perkiraan tentang banyaknya materi di dalam alam semesta berdasarkan efek gravitasi selalu menunjukkan bahwa sebenarnya ada jauh lebih banyak materi daripada materi yang dapat diamati secara langsung. Terlebih lagi, adanya materi gelap dapat menyelesaikan banyak ketidakkonsistenan dalam teori dentuman dahsyat.

Sebagian besar massa di alam semesta dipercaya berada dalam bentuk ini. Menentukan sifat dari materi gelap juga dikenal sebagai masalah materi gelap atau masalah hilangnya massa, dan merupakan salah satu masalah penting dalam kosmologi modern.

Pertanyaan tentang adanya materi gelap mungkin tampak tidak relevan dengan keberadaan kita di bumi. Akan tetapi, ada atau tidaknya materi gelap ini dapat menentukan takdir terakhir dari alam semesta. Kita mengetahui bahwa sekarang alam semesta mengalami pengembangan karena cahaya dari benda langit yang jauh menunjukkan adanya pergeseran merah. Banyaknya materi biasa yang terlihat di alam semesta tidaklah cukup untuk membuat gravitasi menghentikan pengembangan, dan dengan demikian pengembangan akan berlanjut selamanya tanpa adanya materi gelap. Pada prinsipnya, jumlah materi gelap yang cukup di alam semesta dapat menyebabkan pengembangan alam semesta berhenti, atau kebalikannya (yang akhirnya membawa kita pada Big Crunch). Pada prakteknya, sekarang banyak anggapan bahwa gerakan-gerakan alam semesta didominasi oleh komponen lainnya, energi gelap.

References

1. ^ ^{a b} Trimble, V. (1987). "Existence and nature of dark matter in the universe". Annual Review of Astronomy and Astrophysics 25: 425–472. Bibcode 1987ARA&A..25..425T. doi:10.1146/annurev.aa.25.090187.002233.
2. ^ Hinshaw, G. F. (29 January 2010). "What is the universe made of?". Universe 101. NASA/GSFC. http://map.gsfc.nasa.gov/universe/uni_matter.html. Retrieved 2010-03-17.
3. ^ ^{a b c d} Copi, C. J.; Schramm, D. N.; Turner, M. S. (1995). "Big-Bang Nucleosynthesis and the Baryon Density of the Universe". Science 267 (5195): 192–199. arXiv:astro-ph/9407006. Bibcode 1995Sci...267..192C. doi:10.1126/science.7809624. PMID 7809624.
4. ^ Bergstrom, L. (2000). "Non-baryonic dark matter: Observational evidence and detection methods". Reports on Progress in Physics 63 (5): 793–841. arXiv:hep-ph/0002126. Bibcode 2000RPPh...63..793B. doi:10.1088/0034-4885/63/5/2r3.
5. ^ ^{a b c d e f g h i j} Bertone, G.; Hooper, D.; Silk, J. (2005). "Particle dark matter: Evidence, candidates and constraints". Physics Reports 405 (5–6): 279–390. arXiv:hep-ph/0404175. Bibcode 2005PhR...405..279B. doi:10.1016/j.physrep.2004.08.031.
6. ^ Jarosik, N.; et al. (2011). "Seven-Year Wilson Microwave Anisotropy Probe (WMAP) Observations: Sky Maps, Systematic Errors, and Basic Results". Astrophysical Journal Supplement 192 (2): 14. arXiv:1001.4744. Bibcode 2011ApJS..192...14J. doi:10.1088/0067-0049/192/2/14.
7. ^ Siegfried, T. (5 July 1999). "Hidden Space Dimensions May Permit Parallel Universes, Explain Cosmic Mysteries". The Dallas Morning News. http://www.physics.ucdavis.edu/~kaloper/siegfr.txt.
8. ^ Kroupa, P.; et al. (2010). "Local-Group tests of dark-matter Concordance Cosmology: Towards a new paradigm for structure formation". Astronomy and Astrophysics 523: 32–54. arXiv:1006.1647. Bibcode 2010A&A...523A..32K. doi:10.1051/0004-6361/201014892.
9. ^ Raine, D.; Thomas, T. (2001). An Introduction to the Science of Cosmology. IOP Publishing. p. 30. ISBN 0-7503-0405-7. http://www.scribd.com/doc/55748578/34/Baryonic-matter.
10. ^ ^{a b c} Bertone, G.; Merritt, D. (2005). "Dark Matter Dynamics and Indirect Detection". Modern Physics Letters A 20 (14): 1021–1036. arXiv:astro-ph/0504422. Bibcode 2005MPLA...20.1021B. doi:10.1142/S0217732305017391.
11. ^ "Serious Blow to Dark Matter Theories?" (Press release). European Southern Observatory. 18 April 2012. http://www.eso.org/public/news/eso1217/.
12. ^ "The Hidden Lives of Galaxies: Hidden Mass". Imagine the Universe!. NASA/GSFC. http://imagine.gsfc.nasa.gov/docs/teachers/galaxies/imagine/hidden_mass.html.

DARK BARYONS IN GALACTIC HALOS

By: Dr. MARCO RONCADELLI

INFN, Pavia, Italy (roncadelli@pv.infn.it)

Abstract

Primordial nucleosynthesis as well as anisotropies in the cosmic microwave background radiation imply that the total amount of baryons in the Universe largely exceeds the visible contribution, thereby making a strong case for baryonic dark matter. Moreover, certain recent developments lead to a consistent picture of the dark baryon budget in the present-day Universe. 

Accordingly, dark baryons are mostly locked up in galactic halos – which are anyway dominated by nonbaryonic dark matter – and a sizable fraction of them consists of gas clouds. While a priori various forms of baryonic dark matter in galaxies can be conceived, observational constraints rule out most of the possibilities, leaving brown dwarfs and cold gas clouds mostly made of H2 as the only viable candidates (besides supermassive black holes). 

So, it looks natural to suppose that baryonic dark matter in galaxies is accounted for by dark clusters made of brown dwarfs and cold H2 clouds. A few years ago, it was shown that indeed these dark clusters are predicted to populate the outer halos of normal spiral galaxies by the Fall-Rees theory for the formation of globular clusters, which was based on the standard cold dark matter paradigm described in Blumenthal et al. 1984 Nature 311, 517. 

We review the dark cluster formation mechanism, and argue that its qualitative features are expected to remain true even in the contemporary picture of galaxy formation. We also discuss various ramifications of the dark cluster scenario in question, paying particular attention to its observational implications. One of them – the diffuse gamma-ray emission from the Milky Way halo – appears to have been confirmed by the discovery of Dixon et al. 1998 New Astronomy 3, 539. Whether this is actually fact or fiction only the future satellite missions AGILE and GLAST will tell.

http://arxiv.org/pdf/astro-ph/0301537.pdf