Observations of distant supernovae and the mysterious repulsive force driving cosmic acceleration
An unexpected turn in cosmic evolution
For most of the 20th century, cosmologists believed that the Universe's expansion, which began with the Big Bang, would eventually slow down due to the gravitational pull of matter. The central question was whether the Universe would expand forever or eventually contract, depending on its overall mass density. However, in 1998, two independent research teams studying Type Ia supernovae at large redshifts made a stunning discovery: instead of slowing down, the cosmic expansion is accelerating. This unexpected acceleration indicated a new energy component – dark energy, which makes up about 68% of the total energy in the Universe.
The presence of dark energy has fundamentally changed our cosmic worldview. It shows that a repulsive effect operates on a large scale, overshadowing the gravity of matter, causing the expansion to accelerate. The simplest explanation is the cosmological constant (Λ), reflecting vacuum energy in spacetime. However, other theories propose a dynamic scalar field or exotic physics. Although we can measure the effect of dark energy, its essential nature remains one of the greatest mysteries in cosmology, highlighting how much we still do not know about the Universe's future.
2. Evidence of acceleration in observations
2.1 Type Ia supernovae as standard candles
Astronomers use Type Ia supernovae – exploding white dwarfs in binary systems – as "standardized candles." Their peak brightness after calibration is fairly constant, so by comparing observed brightness with redshift we can determine cosmic distances and expansion history. In the late 1990s, the High-z Supernova Search Team (A. Riess, B. Schmidt) and the Supernova Cosmology Project (S. Perlmutter) found that distant supernovae (~z 0.5–0.8) appear dimmer than expected if the Universe were decelerating or steady. Accelerating expansion fits best [1,2].
2.2 CMB and large-scale structure studies
Further WMAP and Planck satellite cosmic microwave background (CMB) anisotropy data have determined precise cosmic parameters showing that all matter (dark + baryonic) makes up only ~31% of the critical density, with the remaining (~69%) composed of mysterious dark energy or "Λ." Large-scale structure studies (e.g., SDSS) observing baryon acoustic oscillations (BAO) support the accelerating expansion hypothesis. All these data agree that in the ΛCDM model about 5% of matter is baryons, ~26% is dark matter, and ~69% is dark energy [3,4].
2.3 Baryon acoustic oscillations and structure growth
Baryon acoustic oscillations (BAO), observed in the large-scale distribution of galaxies, act as a "standard ruler" to measure expansion at different times. Their patterns show that over the past ~several billion years the Universe's expansion is accelerating, so structure growth is slower than expected from matter domination alone. All different data sources point to the same conclusion: there is an accelerating component overcoming matter's deceleration.
3. The cosmological constant: the simplest explanation
3.1 Einstein's Λ and vacuum energy
Albert Einstein introduced the cosmological constant Λ in 1917 to obtain a static Universe. When Hubble discovered that the Universe is expanding, Einstein abandoned Λ, calling it "the biggest mistake." Paradoxically, Λ returned as the main candidate for the source of acceleration: vacuum energy, whose equation of state p = -ρ c² creates negative pressure and a repulsive gravitational effect. If Λ is truly constant, the Universe will approach exponential expansion in the future, as matter density becomes negligible.
3.2 Scale and the “Fine-tuning” Problem
The observed dark energy (Λ) density is about ~ (10-12 GeV)4, while quantum field theory predicts a much larger vacuum energy. This cosmological constant problem asks: why is the measured Λ so small compared to Planck scale predictions? Attempts to find what cancels that huge amount have yet to find a convincing explanation. This is one of the biggest physics "fine-tuning" challenges.
4. Dynamic Dark Energy: Quintessence and Alternatives
4.1 Quintessence Fields
Instead of a constant Λ, some scientists propose a dynamic scalar field φ with a potential V(φ) that changes over time – often called "quintessence". Its equation of state w = p/ρ can differ from -1 (as it should be for a pure cosmological constant). Observations show w ≈ -1 ± 0.05, still allowing for slight deviation. If w changed over time, we might learn about a different expansion rate in the future. However, no solid signs of temporal variation have been seen so far.
4.2 “Phantom” Energy or k-essence
Some models allow w < -1 ("phantom" energy), leading to the "Big Rip", where expansion eventually tears apart even atoms. Or "k-essence" introduces non-conformal kinetic terms. This is speculative, and when evaluating supernova, BAO, and CMB data, nothing has yet shown a clear advantage over the simple, nearly constant Λ.
4.3 Modified gravity
Another approach is to modify general relativity on large scales instead of introducing dark energy. For example, extra dimensions, f(R) theories, or brane world models can produce apparent acceleration. However, reconciling precise Solar System tests with cosmological data is difficult. So far, no attempts have clearly outperformed the simple Λ theory in the broader observational context.
5. The “Why now?” question and the coincidence problem
5.1 Cosmic Coincidence
Dark energy began to dominate only a few billion years ago – why is the Universe accelerating now, and not earlier or later? This is called the "coincidence problem", suggesting that perhaps the anthropic principle ("intelligent observers arise ~when matter and Λ are of similar order") explains this coincidence. The standard ΛCDM does not solve this by itself but accepts it as part of the anthropic context.
5.2 The Anthropic Principle and the Multiverse
Vien explains that if Λ were much larger, structures wouldn't form before acceleration prevented matter from clumping. If Λ were negative or different, other evolutionary conditions would arise. The anthropic principle states that we observe Λ of exactly the size that allows galaxies and observers to form. With multiverse ideas, it can be argued that different "bubbles" (Universes) have different vacuum energy values, and we ended up in this one due to favorable conditions.
6. Future prospects of the Universe
6.1 Eternal acceleration?
If dark energy is truly a constant Λ, the Universe will undergo exponential expansion in the future. Galaxies that are not gravitationally bound (not part of the local group) will move beyond our cosmological horizon, eventually "disappearing" from view and leaving us in a "lonely Universe" where only local merged galaxies remain.
6.2 Other scenarios
- Dynamic quintessence: if w > -1, expansion will be slower than exponential, close to a de Sitter state, but not as strong.
- Phantom energy (w < -1): Could end with the "Big Rip," where expansion overcomes even atomic bonds. Current data somewhat contradict a strong "phantom" scenario but do not exclude a small w < -1.
- Vacuum decay: If the vacuum is only metastable, it could suddenly transition to a lower energy state – a fatal event in the context of physics. However, so far this is only speculation.
7. Current and future research
7.1 Extremely precise cosmological projects
Projects such as DES (Dark Energy Survey), eBOSS, Euclid (ESA), and the upcoming Vera C. Rubin (LSST) observatory will study billions of galaxies, measuring the expansion history through supernovae, BAO, weak lensing, and structure growth. It is expected to determine the equation of state parameter w to about ~1% accuracy to check if it really equals -1. If a deviation in w is detected, it would indicate dynamic dark energy.
7.2 Gravitational waves and multi-messenger astronomy
In the future, the detection of gravitational waves from standard "sirens" (neutron star mergers) will allow independent measurements of cosmic distance and expansion. Combined with electromagnetic signals, this will further refine the evolution of dark energy. Also, 21 cm wavelength measurements during the cosmic dawn period may help study expansion at greater distances and increase our knowledge of dark energy behavior.
7.3 Theoretical breakthroughs?
Solving the cosmological constant problem or discovering the microphysical basis of quintessence may be possible if the perspectives of quantum gravity or string theory improve. New symmetry principles (e.g., supersymmetry, which, unfortunately, has not yet been detected at the LHC), or anthropic arguments may also explain why dark energy is so small. If "dark energy excitations" or an additional "fifth force" were detected, it would completely change our understanding. So far, unfortunately, observations do not support this.
8. Conclusion
Dark energy is one of the greatest mysteries in cosmology: the repulsive component responsible for the accelerating expansion of the Universe, unexpectedly discovered at the end of the 20th century by studying distant type Ia supernovae. Numerous additional data ( CMB, BAO, lensing, structure growth) confirm that dark energy makes up ~68–70% of the Universe's energy, based on the standard ΛCDM model. The simplest option is the cosmological constant, but it poses challenges such as the cosmological constant problem and the “coincidence” issues.
These ideas (quintessence, modified gravity, holographic concept) are still quite speculative and do not have as well-tested empirical support as the nearly stable Λ. Upcoming observatories – Euclid, LSST, Roman Space Telescope – will significantly refine our knowledge of the equation of state in the coming years and may clarify whether the acceleration rate changes over time or hints at new physics. Understanding what dark energy is will not only determine the fate of the Universe (whether eternal expansion, “big rip,” or other endings) but also help us understand how quantum fields, gravity, and spacetime itself interact. Thus, solving the mystery of dark energy is a key step in the cosmic detective story that tells how the Universe evolves, persists, and perhaps eventually disappears from our view as cosmic expansion accelerates.
References and further reading
- Riess, A. G., et al. (1998). “Observational evidence from supernovae for an accelerating universe and a cosmological constant.” The Astronomical Journal, 116, 1009–1038.
- Perlmutter, S., et al. (1999). “Measurements of Ω and Λ from 42 high-redshift supernovae.” The Astrophysical Journal, 517, 565–586.
- Planck Collaboration (2018). “Planck 2018 results. VI. Cosmological parameters.” Astronomy & Astrophysics, 641, A6.
- Weinberg, S. (1989). “The cosmological constant problem.” Reviews of Modern Physics, 61, 1–23.
- Frieman, J. A., Turner, M. S., & Huterer, D. (2008). “Dark energy and the accelerating universe.” Annual Review of Astronomy and Astrophysics, 46, 385–432.