Dark Energy and Alternative ideas

Some theorists think that dark energy and cosmic acceleration are a failure of general relativity on very large scales, larger than superclusters. It is a tremendous extrapolation to think that our law of gravity, which works so well in the solar system, should work without correction on the scale of the universe. 

Most attempts at modifying general relativity, however, have turned out to be either equivalent to theories of quintessence, or inconsistent with observations. It is of interest to note that if the equation for gravity were to approach r instead of r² at large, intergalactic distances, then the acceleration of the expansion of the universe becomes a mathematical artifact, negating the need for the existence of Dark Energy.

Alternative ideas for dark energy have come from string theory, brane cosmology and the holographic principle, but have not yet proved as compelling as quintessence and the cosmological constant. On string theory, an article in the journal Nature described:

String theories, popular with many particle physicists, make it possible, even desirable, to think that the observable universe is just one of 10⁵⁰⁰ universes in a grander multiverse, says [Leonard Susskind, a cosmologist at Stanford University in California]. The vacuum energy will have different values in different universes, and in many or most it might indeed be vast. But it must be small in ours because it is only in such a universe that observers such as ourselves can evolve.

Paul Steinhardt in the same article criticizes string theory's explanation of dark energy stating "...Anthropics and randomness don't explain anything... I am disappointed with what most theorists are willing to accept".

In a rather radical departure, an article in the open access journal, Entropy, by Professor Paul Gough, put forward the suggestion that information energy must make a significant contribution to dark energy and that this can be shown by referencing the equation of the state of information in the universe. 

Yet another, "radically conservative" class of proposals aims to explain the observational data by a more refined use of established theories rather than through the introduction of dark energy, focusing, for example, on the gravitational effects of density inhomogeneities, or on consequences of electroweak symmetry breaking in the early universe.

Kunjungi Juga:

Central Research for Dark Energy & Dark Matter 

Wikipedia

Memahami Konstanta Kosmologi

Cosmological constant

 

Main article: Cosmological constant

For more details on this topic, see Equation of state (cosmology).

The simplest explanation for dark energy is that it is simply the "cost of having space": that is, a volume of space has some intrinsic, fundamental energy. This is the cosmological constant, sometimes called Lambda (hence Lambda-CDM model) after the Greek letter Λ, the symbol used to mathematically represent this quantity. 

Since energy and mass are related by E = mc², Einstein's theory of general relativity predicts that it will have a gravitational effect. It is sometimes called a vacuum energy because it is the energy density of empty vacuum. In fact, most theories of particle physics predict vacuum fluctuations that would give the vacuum this sort of energy. 

This is related to the Casimir Effect, in which there is a small suction into regions where virtual particles are geometrically inhibited from forming (e.g. between plates with tiny separation). 

The cosmological constant is estimated by cosmologists to be on the order of 10⁻²⁹g/cm³, or about 10⁻¹²⁰ in reduced Planck units. However, particle physics predicts a natural value of 1 in reduced Planck units, a large discrepancy which is still lacking in explanation.

The cosmological constant has negative pressure equal to its energy density and so causes the expansion of the universe to accelerate. The reason why a cosmological constant has negative pressure can be seen from classical thermodynamics; Energy must be lost from inside a container to do work on the container.

A change in volume dV requires work done equal to a change of energy −p dV, where p is the pressure. But the amount of energy in a box of vacuum energy actually increases when the volume increases (dV is positive), because the energy is equal to ρV, where ρ (rho) is the energy density of the cosmological constant. Therefore, p is negative and, in fact, p = −ρ.

A major outstanding problem is that most quantum field theories predict a huge cosmological constant from the energy of the quantum vacuum, more than 100 orders of magnitude too large. This would need to be cancelled almost, but not exactly, by an equally large term of the opposite sign. 

Some supersymmetric theories require a cosmological constant that is exactly zero, which does not help. The present scientific consensus amounts to extrapolating the empirical evidence where it is relevant to predictions, and fine-tuning theories until a more elegant solution is found. Philosophically, our most elegant solution may be to say that if things were different, we would not be here to observe anything — the anthropic principle.

Technically, this amounts to checking theories against macroscopic observations. Unfortunately, as the known error-margin in the constant predicts the fate of the universe more than its present state, many such "deeper" questions remain unknown.

Another problem arises with inclusion of the cosmic constant in the standard model: i.e., the appearance of solutions with regions of discontinuities (see classification of discontinuities for three examples) at low matter density.

Discontinuity also affects the past sign of the pressure assigned to the cosmic constant, changing from the current negative pressure to attractive, as one looks back towards the early Universe. 

A systematic, model-independent evaluation of the supernovae data supporting inclusion of the cosmic constant in the standard model indicates these data suffer systematic error. The supernovae data are not overwhelming evidence for an accelerating Universe expansion which may be simply gliding.

A numerical evaluation of WMAP and supernovae data for evidence that our local group exists in a local void with poor matter density compared to other locations, uncovered possible conflict in the analysis used to support the cosmic constant.

These findings should be considered shortcomings of the standard model, but only when a term for vacuum energy is included.

In spite of its problems, the cosmological constant is in many respects the most economical solution to the problem of cosmic acceleration. One number successfully explains a multitude of observations. Thus, the current standard model of cosmology, the Lambda-CDM model, includes the cosmological constant as an essential feature.

Sumber:

Wikipedia

Kunjungi Juga:

Central Research for Dark Energy & Dark Matter

GAYA

Gaya-gaya di Alam

"Jika kita bergetar dan tertarik oleh seseorang percayalah itu bukan Gravitasi"

-Arip-

Gaya-gaya fundamental

Berbagai macam gaya yang diamati di alam dapat dijelaskan lewat 4 interaksi dasar yang terjadi antara partikel-partikel elementer:

1. Gaya gravitasi
2. Gaya elektromagnetik
3. Gaya nuklir kuat (juga dinamakan gaya hadronik)
4. Gaya nuklir lemah


Gaya gravitasi antara bumi dan sebuah benda di dekat permukaan bumi adalah berat benda.

Gaya gravitasi yang dikerjakan oleh Matahari pada bumi dan planet-planet lain bertanggung jawab untuk mempertahankan planet-planet dalam orbitnya mengelilingi Matahari.

Demikian pula, gaya gravitasi yang dikerjakan oleh Bumi pada bulan, menjaga Bulan dalam orbitnya yang mendekati lingkaran mengelilingi bumi.

Gaya gravitasi yang dikerjakan oleh bulan dan matahari pada lautan di bumi bertanggung jawab terhadap peristiwa pasang surut.

Gaya elektromagnetik mencakup gaya-gaya listrik dan gaya magnetik.

Sebuah contoh yang terkenal tentang gaya listrik adalah tarikan antara potongan-potongan kertas kecil dan sisir yang telah diberi muatan listrik dengan digosokan pada rambut.

Walaupun gaya magnetik yang terkenal antara sebuah magnet dan benda-benda besi tampaknya sangat berbeda dari gaya listrik, namun sebetulnya gaya magnetik muncul bila muatan listrik dalam keadaan bergerak.

Gaya elektromagnetik antara partikel elementer yang bermuatan sangat lebih besar daripada gaya gravitasi di antara partikel elementer sehingga gaya gravitasi dapat hampir selalu diabaikan.

Sebagai contoh, gaya tolak elektrostatik antara dua proton berorde 10^36 (Sepuluh pangkat 36) kali tarikan gravitasi antara dua proton.

Gaya nuklir kuat terjadi antara partikel-partikel elementer yang dinamakan hadron, yang di dalamnya termasuk proton dan neutron, unsur pokok inti atom.

Gaya ini bertanggung jawab untuk mengikat inti atom menjadi satu.

Sebagai contoh, kedua proton dalam atom helium terikat lewat gaya nuklir yang kuat, yang lebih dari mengimbangi tolakan elektrostatika proton.

Namun, gaya nuklir kuat mempunyai jangkauan sangat pendek. Gaya ini berkurang dengan cepat bersamaan dengan pemisahan partikel-partikel, dan dapat diabaikan jika partikel-partikel terpisah sejauh beberapa diameter nuklir.

Gaya nuklir lemah, yang juga mempunyai jangkauan pendek, terjadi antara elektron dan proton atau neutron.

Gaya inilah yang bertanggung jawab untuk sejenis peluruhan radioaktif tertentu yang dinamakan peluruhan beta.

Gaya-gaya fundamental bekerja di antara partikel-partikel yang terpisah dalam ruang.

Konsep ini dihubungkan dengan aksi pada jarak.

Newton menganggap, aksi pada suatu jarak sebagai suatu cacat dalam teori gravitasinya, tetapi beliau menolak memberikan hipotesis lain.

Dan akhirnya Prof. Newton menuliskan hal berikut ini:

Sir Isaac Newton, surat ketiga kepada Bentley (25 Februari 1692). J. Dodsley, London, 1756.

Tidaklah dapat dibayangkan bahwa benda mati, bahan kasar, tanpa perantara sesuatu yang lain, yang bukan materi, bekerja pada dan mempengaruhi benda lain tanpa saling kontak.

Seperti yang terjadi bila pengertian Gravitasi dalam pengertian Epicurus, adalah penting dan inheren di dalamnya.

Ini adalah satu sebab mengapa saya menginginkan Anda tidak menganggap swadaya gravitasi berasal dari saya.

Bahwa gravitasi haruslah swadaya, inheren dan penting bagi bahan, agar satu benda dapat bekerja pada benda lain pada suatu jarak lewat ruang hampa, tanpa perantaraan apa pun yang lain, oleh dan lewat aksi mereka dan gaya dapat diteruskan dari satu ke yang lainnya.

Untuk saya suatu kemustahilan yang demikian besarnya sehingga saya percaya tidak ada orang yang mempunyai kemampuan berpikir yang baik dalam masalah filosofi dapat jatuh kedalamnya.


Sumber:
Prof. Paul A. Tipler, Ph.D.
Professor of Physics, University of California at Berkeley.
Unduh Buku Physics:  http://www.jalurcepat.com/vzoq6p9r9nin/Physics__5_Ed._-_Paul_A._Tipler.pdf.htm