Physics / Fizik

Permanent URI for this collectionhttps://hdl.handle.net/11147/6

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Author: search.filters.author.Erdem, RecaiScopus Q: search.filters.scopusQuartile.Q2

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  • Article
    It Is Sufficient To Set the Cosmological Constant To Zero or To a Small Number at an Initial Time
    (TUBITAK, 2016) Erdem, Recai
    I point out a simple but usually overlooked fact about the cosmological constant problem: to solve the cosmological constant problem it is sufficient to find a symmetry or mechanism that sets the cosmological constant to zero or to a tiny value at some time in the past, provided that general relativity is the relevant theory of gravity, and the energy-momentum tensor (excluding the part of the form of a cosmological constant) is conserved. The relevant symmetry or mechanism need not be applicable today. Any additional cosmological constant term induced by a phase transition in the energy-momentum tensor in this case is compensated by a shift in the cosmological constant term of gravitational origin.
  • Article
    Citation - WoS: 7
    Citation - Scopus: 8
    A Way To Get Rid of Cosmological Constant and Zero-Point Energy Problems of Quantum Fields Through Metric Reversal Symmetry
    (IOP Publishing Ltd., 2008) Erdem, Recai
    In this paper, a framework is introduced to remove the huge discrepancy between the empirical value of the cosmological constant and the contribution to the cosmological constant predicted from the vacuum energy of quantum fields. An extra-dimensional space with metric reversal symmetry and R2 gravity (that reduces to the usual R gravity after integration over extra dimensions) is considered to this end. The resulting four-dimensional energy-momentum tensor (obtained after integration over extra dimensions) consists of terms that contain off-diagonally coupled pairs of Kaluza-Klein modes. This, in turn, generically results in the vanishing of the vacuum expectation value of the energy-momentum tensor for quantum fields, and offers a way to solve the problem of huge contribution of quantum fields to the vacuum energy density.
  • Article
    Citation - WoS: 8
    Citation - Scopus: 9
    A Symmetry for the Vanishing Cosmological Constant
    (IOP Publishing Ltd., 2007) Erdem, Recai
    Two different realizations of a symmetry principle that impose a zero cosmological constant in an extra-dimensional set-up are studied. The symmetry is identified by multiplication of the metric by minus one. In the fist realization of the symmetry this is provided by a symmetry transformation that multiplies the coordinates by the imaginary number i. In the second realization this is accomplished by a symmetry transformation that multiplies the metric tensor by minus one. In both realizations of the symmetry the requirement of the invariance of the gravitational action under the symmetry selects out the dimensions given by D ≤ 2(2n + 1), n ≤ 0, 1, 2..., and forbids a bulk cosmological constant. Another attractive aspect of the symmetry is that it seems to be more promising for quantization when compared to the usual scale symmetry. The second realization of the symmetry principle is more attractive in that it is possible to make a possible brane cosmological constant zero in a simple way by using the same symmetry, and the symmetry may be identified by reflection symmetry in extra dimensions.