Borde–Guth–Vilenkin theorem

The Borde–Guth–Vilenkin theorem, or the BGV theorem, is a theorem in physical cosmology which deduces that any universe that has, on average, been expanding throughout its history cannot be infinite in the past but must have a past spacetime boundary.^[1] The theorem does not assume any specific mass content of the universe and it does not require gravity to be described by Einstein field equations. It is named after the authors Arvind Borde, Alan Guth and Alexander Vilenkin, who developed its mathematical formulation in 2003.^[2][3] The BGV theorem is also popular outside physics, especially in religious and philosophical debates.^[3][4][5]

Formal definition

Derivation

For flat spacetime

Here is an example of derivation of the BGV theorem for an expanding homogeneous isotropic flat universe (in units of speed of light c=1).^[6] Which is consistent with ΛCDM model, the current model of cosmology. However, this derivation can be generalized to an arbitrary space-time with no appeal to homogeneity or isotropy.^[6]

The Friedmann–Lemaître–Robertson–Walker metric is given by

${\displaystyle ds=dt^{2}-a^{2}(t)dx_{i}dx^{i}}$ ,

where t is time, xⁱ (i=1,2,3) are the spatial coordinates and a(t) is the scale factor. Along a timeline geodesic x_i = constant, we can consider the universe to be filled with comoving particles. For an observer with proper time τ following the world line x^μ(τ), has a 4-momentum ${\displaystyle P^{\mu }=mdx^{\mu }/d\tau =(E,\mathbf {p} )}$ , where ${\displaystyle E={\sqrt {p^{2}+m^{2}}}}$  is the energy, m is the mass and p=|p| the magnitude of the 3-momentum.

From the geodesic equation of motion, it follows that ${\displaystyle p(t)=p_{\rm {f}}\;a(t_{\rm {f}})/a(t)}$  where p_f is the final momentum at time t_f. Thus

${\displaystyle \int _{t_{\rm {i}}}^{t_{\rm {f}}}H(\tau )d\tau =\int _{a(t_{\rm {i}})}^{a(t_{\rm {f}})}{\frac {mda}{\sqrt {m^{2}a^{2}+p^{2}a(t_{\rm {f}})}}}=F(\gamma _{\rm {f}})-F(\gamma _{\rm {i}})\leq F(\gamma _{\rm {f}})}$ ,

where t_i is an initial time, ${\displaystyle H={\dot {a}}/a}$  is the Hubble parameter, and

${\displaystyle F(\gamma )={\frac {1}{2}}\ln \left({\frac {\gamma +1}{\gamma -1}}\right)}$ ,

γ being the Lorentz factor. For any non-comoving observer γ>1 and F(γ)>0.

The expansion rate averaged over the observer world line can be defined as

${\displaystyle H_{\rm {av}}={\frac {1}{\tau _{\rm {f}}-\tau _{\rm {i}}}}\int _{t_{\rm {i}}}^{t_{\rm {f}}}H(\tau )d\tau }$ .

Assuming ${\displaystyle H_{\rm {av}}>0}$  it is follows that

${\displaystyle \tau _{\rm {f}}-\tau _{\rm {i}}\leq {\frac {F(\gamma _{\rm {f}})}{H_{\rm {av}}}}}$ .

Thereby any non-comoving past-directed timelike geodesic satisfying the condition ${\displaystyle H_{\rm {av}}>0}$ , must have a finite proper length, and so must be past-incomplete.

Limitations and criticism

Alternative models, where the average expansion of the universe throughout its history does not hold, have been proposed under the notions of emergent spacetime, eternal inflation, and cyclic models. Vilenkin and Audrey Mithani have argued that none of these models escape the implications of the theorem.^[7] In 2017, Vilenkin stated that he does not think there are any viable cosmological models that escape the scenario.^[8]

Sean M. Carroll argues that the theorem only applies to classical spacetime, and may not hold under consideration of a complete theory of quantum gravity. He added that Alan Guth, one of the co-authors of the theorem, disagrees with Vilenkin and believes that the universe had no beginning.^[9][10] Vilenkin argues that the Carroll-Chen model constructed by Carroll and Jennie Chen, and supported by Guth, to elude the BGV theorem's conclusions persists to indicate a singularity in the history of the universe as it has a reversal of the arrow of time in the past.^[11]

Joseph E. Lesnefsky, Damien A. Easson and Paul Davies constructed an uncountable infinite class of classical solutions which have ${\displaystyle H_{\rm {av}}\geq 0}$  and are geodesically complete.^[12] The authors claim that the geodesic incompleteness of inflationary spacetime is still an open issue. Furthermore, there are examples of infinite cyclic models solving the problem of unbounded entropy growth which are geodesically complete.^[13] In both of these studies, the authors argue that the previous investigations often did not use mathematically precise formulations of the BGV theorem and thus reached incomplete conclusions.

Use in theology

Vilenkin has also written about the religious significance of the BGV theorem. In October 2015, Vilenkin responded to arguments made by theist William Lane Craig and the New Atheism movement regarding the existence of God. Vilenkin stated "What causes the universe to pop out of nothing? No cause is needed."^[6]

See also

• Kalam cosmological argument
• Gibbons–Hawking–York boundary term
• Gibbons–Hawking effect

References

1. Perlov, Delia; Vilenkin, Alexander (7 August 2017). Cosmology for the Curious. Cham, Switzerland: Springer. pp. 330–31. ISBN 978-3319570402.
2. Borde, Arvind; Guth, Alan H.; Vilenkin, Alexander (15 April 2003). "Inflationary space-times are incomplete in past directions". Physical Review Letters. 90 (15): 151301. arXiv:gr-qc/0110012. Bibcode:2003PhRvL..90o1301B. doi:10.1103/PhysRevLett.90.151301. PMID 12732026. S2CID 46902994.
3. Perlov, Delia; Vilenkin, Alexander (7 August 2017). Cosmology for the Curious. Cham, Switzerland: Springer. pp. 330–31. ISBN 978-3319570402.
4. Copan, Paul; Craig, William Lane (2017-11-16). The Kalam Cosmological Argument, Volume 2: Scientific Evidence for the Beginning of the Universe. Bloomsbury Publishing USA. ISBN 9781501335891.
5. Nagasawa, Y. (2012-07-25). Scientific Approaches to the Philosophy of Religion. Springer. ISBN 9781137026019.
6. Vilenkin, Alexander (2015-10-23). . Inference. 1 (4). Archived from the original on 27 December 2023.
7. Mithani, Audrey; Vilenkin, Alexander (20 April 2012). "Did the universe have a beginning?". arXiv:1204.4658 [hep-th].
8. Alexander Vilenkin, "The Beginning of the Universe" in The Kalam Cosmological Argument: Volume 2, Bloomsbury, 2017, pp. 150–158
9. Carroll, Sean (2014-02-24). "Post-Debate Reflections". Sean Carroll Blog. from the original on 2014-02-25. Retrieved 2019-11-19.
10. Carroll, Sean M. (2018-06-04). "Why Is There Something, Rather Than Nothing?". arXiv:1802.02231 [physics.hist-ph].
11. Vilenkin, Alexander (2013). "Arrows of time and the beginning of the universe". Physical Review D. 88 (4): 043516. arXiv:1305.3836. Bibcode:2013PhRvD..88d3516V. doi:10.1103/PhysRevD.88.043516. S2CID 119213877.
12. Lesnefsky, J. E.; Easson, D. A.; Davies, P. C. W. (2023). "Past-completeness of inflationary spacetimes". Physical Review D. 107 (4): 044024. arXiv:2207.00955. doi:10.1103/PhysRevD.107.044024.
13. Pavlović, Petar; Sossich, Marko (2024). "Geodesically complete cyclic cosmologies and entropy". European Physical Journal C. 84 242. arXiv:2305.06719. doi:10.1140/epjc/s10052-024-12621-z.

Further reading

• Calcagni, Gianluca (2017-01-06). Classical and Quantum Cosmology. Springer. ISBN 9783319411279.