Dark energy

Within physical cosmology and astronomy, dark energy represents a hypothesized form of energy influencing the universe at its most expansive scales. Its principal impact is to propel the accelerating expansion of the cosmos. Concurrently, it retards the pace of cosmic structure formation. Under the premise that the lambda-CDM cosmological model is accurate, dark energy constitutes the predominant component of the universe, accounting for 68% of the total energy within the observable universe today. In contrast, dark matter and ordinary (baryonic) matter contribute 27% and 5%, respectively, with other constituents like neutrinos and photons being largely insignificant. The density of dark energy is exceptionally low, measured at approximately 7×10−30 g/cm3 (equivalent to 6×10−10 J/m3 in mass-energy), which is considerably lower than the densities of ordinary or dark matter found within galaxies. Nevertheless, its uniform distribution throughout space enables it to dominate the universe's total mass-energy content.

In physical cosmology and astronomy, dark energy is a proposed form of energy that affects the universe on the largest scales. Its primary effect is to drive the accelerating expansion of the universe. It also slows the rate of structure formation. Assuming that the lambda-CDM model of cosmology is correct, dark energy dominates the universe, contributing 68% of the total energy in the present-day observable universe while dark matter and ordinary (baryonic) matter contribute 27% and 5%, respectively, and other components such as neutrinos and photons are nearly negligible. Dark energy's density is very low: 7×10⁻³⁰ g/cm§910§ (§1415§×§1617§−10 J/m§1920§ in mass-energy), much less than the density of ordinary matter or dark matter within galaxies. However, it dominates the universe's mass–energy content because it is uniform across space.

Initial observational evidence supporting the existence of dark energy emerged from supernova measurements. Type Ia supernovae exhibit a consistent luminosity, rendering them reliable indicators for cosmic distance measurements. A comparison of these distances with their respective redshifts, which quantify the recession velocity of the supernovae, reveals an accelerating expansion of the universe. Before this discovery, the prevailing scientific hypothesis posited that the gravitational attraction exerted by matter and energy within the universe would progressively decelerate its expansion. Subsequent to the identification of accelerating expansion, multiple independent lines of evidence have been uncovered, further substantiating the presence of dark energy.

The precise nature of dark energy continues to be enigmatic, prompting the development of numerous theoretical explanations. Primary candidates include a cosmological constant, which signifies a uniform energy density pervading space, and scalar fields, which are dynamic entities possessing energy densities that fluctuate across time and space, exemplified by quintessence or moduli. While a cosmological constant maintains invariance across temporal and spatial dimensions, scalar fields are inherently variable. Additional hypotheses encompass interacting dark energy , an observational artifact, cosmological coupling, and shockwave cosmology .

Historical Context of Discovery and Prior Speculations

Einstein's Cosmological Constant

The cosmological constant represents the most straightforward theoretical candidate for dark energy. This constant term can be incorporated into the Einstein field equations of general relativity. When conceptualized as a "source term" within the field equation, it can be interpreted as equivalent to the mass of empty space, which could theoretically be positive or negative, or as "vacuum energy."

Einstein initially proposed the cosmological constant as a mechanism to derive a solution for the gravitational field equations that would yield a static universe, essentially employing dark energy to counteract gravitational forces. Einstein designated the cosmological constant with the symbol Λ (capital lambda). Einstein posited that the cosmological constant necessitated 'empty space assuming the role of gravitating negative masses distributed throughout interstellar space'.

This mechanism exemplified fine-tuning, and subsequent understanding revealed that Einstein's static universe model would inherently be unstable; localized inhomogeneities would inevitably precipitate either an uncontrolled expansion or contraction of the cosmos. The equilibrium state is unstable: a slight expansion of the universe would release vacuum energy, thereby inducing further expansion. Conversely, a universe undergoing a minor contraction would persist in its contraction. Einstein theorized that "empty space" could intrinsically possess its own energy. As this energy is an inherent characteristic of space, it would not diminish with cosmic expansion. The emergence of additional space would consequently manifest more of this intrinsic spatial energy, leading to accelerated expansion. Such perturbations are unavoidable, given the heterogeneous distribution of matter across the universe. Furthermore, observations conducted by Edwin Hubble in 1929 demonstrated that the universe is expanding, rather than remaining static. Einstein reportedly characterized his oversight in anticipating a dynamic universe, as opposed to a static one, as his most significant error.

Inflationary Dark Energy

In 1980, Alan Guth and Alexei Starobinsky independently proposed that a negative pressure field, conceptually akin to dark energy, could have propelled cosmic inflation in the nascent universe. Inflation posits that a repulsive force, qualitatively similar to dark energy, caused an immense and exponential expansion of the universe during its earliest moments. This expansion constitutes a fundamental characteristic of most contemporary Big Bang models. Nevertheless, inflation is theorized to have transpired at a significantly greater energy density than the dark energy currently observed, concluding entirely when the universe was merely a fraction of a second old. The precise relationship, if any, between dark energy and inflation remains undefined. Historically, even after the acceptance of inflationary models, the cosmological constant was considered inconsequential to the present-day universe.

Late-Time Dark Energy

Almost all inflationary models predict that the universe's total matter and energy density should approximate the critical density. Throughout the 1980s, cosmological research predominantly focused on models positing a critical density composed solely of matter, typically comprising 95% cold dark matter (CDM) and 5% ordinary matter (baryons). While these models successfully simulated realistic galaxy and cluster formation, issues emerged in the late 1980s. Specifically, the models necessitated a Hubble constant value lower than observational preferences and under-predicted large-scale galaxy clustering. These challenges intensified following the 1992 discovery of cosmic microwave background anisotropy by the COBE spacecraft, prompting active investigation into several modified CDM models through the mid-1990s, including the Lambda-CDM model and a mixed cold/hot dark matter model. Direct evidence for dark energy first materialized in 1998 from supernova observations, which revealed accelerated cosmic expansion, as reported by Riess et al. and Perlmutter et al.. Consequently, the Lambda-CDM model ascended to prominence. Soon thereafter, independent observations corroborated the existence of dark energy: in 2000, the BOOMERanG and Maxima cosmic microwave background experiments detected the first acoustic peak, indicating that the total matter and energy density approaches 100% of the critical density. Subsequently, in 2001, the 2dF Galaxy Redshift Survey provided robust evidence that matter density constitutes approximately 30% of the critical density. The substantial disparity between these findings supports the presence of a smooth dark energy component accounting for the remainder. More precise measurements from WMAP between 2003 and 2010 have further substantiated the standard model and yielded refined values for key cosmological parameters.

Cosmologist Michael S. Turner coined the term "dark energy" in 1998 for a collaborative paper authored with Saul Perlmutter and Martin White.

Nature

The fundamental nature of dark energy remains more speculative than that of dark matter, with many aspects still theoretical. Dark energy is hypothesized to be highly homogeneous and of low density, and it is not known to interact via any fundamental forces apart from gravity. Given its rarefied and insubstantial character—approximately 10⁻²⁷ kg/m³—its detection in laboratory experiments is improbable. Despite its extreme dilution, dark energy exerts a profound influence on the universe, constituting 68% of the universal density, because it is believed to uniformly permeate otherwise empty space.

Vacuum energy, defined as the transient generation and mutual annihilation of particle–antiparticle pairs within a timeframe consistent with Heisenberg's energy–time uncertainty principle, has frequently been proposed as a primary contributor to dark energy. The mass–energy equivalence principle of general relativity dictates that vacuum energy should exert a gravitational force. Consequently, vacuum energy is anticipated to contribute to the cosmological constant, which directly influences the universe's accelerated expansion. Nevertheless, the cosmological constant problem highlights a substantial discrepancy between the empirically observed vacuum energy density and the exceedingly large theoretical zero-point energy derived from quantum field theory; this fundamental issue persists without resolution.

Irrespective of its precise composition, dark energy must possess substantial negative pressure to account for the observed accelerated expansion of the cosmos. General relativity posits that a substance's internal pressure, alongside its mass density, contributes to its gravitational influence on other entities. This phenomenon arises because the stress–energy tensor, which encompasses both the energy (or matter) density and the pressure of a substance, is the fundamental physical quantity responsible for generating gravitational effects. Within the framework of the Friedmann–Lemaître–Robertson–Walker metric, a pervasive, strong, constant negative pressure (i.e., tension) throughout the universe demonstrably induces an acceleration in cosmic expansion if the universe is already expanding, or a deceleration in contraction if it is already contracting. This accelerative expansion is occasionally termed "gravitational repulsion."

Technical Definition

Standard cosmological models identify three primary constituents of the universe: matter, radiation, and dark energy. Matter is characterized by an energy density that scales inversely with the cube of the cosmic scale factor, specifically ρ ∝ a⁻³. Conversely, radiation exhibits an energy density that scales inversely with the fourth power of the scale factor (ρ ∝ a⁻⁴). This scaling behavior can be intuitively grasped: if an ordinary particle resides within a cubic volume, doubling the length of the box's edge reduces its density, and consequently its energy density, by a factor of eight (2§1617§). For radiation, the reduction in energy density is more pronounced, as an expansion in spatial distance not only dilutes the energy but also induces a redshift, thereby diminishing the energy per photon (refer to the Planck relation).

The third component, dark energy, represents an inherent characteristic of space itself, maintaining a constant energy density irrespective of the volume's dimensions (ρ ∝ a§56§). Consequently, unlike conventional matter, dark energy's density remains unaffected by cosmic expansion.

Temporal Evolution of Cosmic Expansion

Precise measurements of the universe's expansion are essential for comprehending the temporal and spatial variations in its expansion rate. Within general relativity, the evolution of this expansion rate is derived from the universe's curvature and the cosmological equation of state, which defines the interrelationship among temperature, pressure, and the combined densities of matter, energy, and vacuum energy within any given spatial region. A significant endeavor in contemporary observational cosmology involves determining the equation of state for dark energy. The incorporation of the cosmological constant into the standard Friedmann–Lemaître–Robertson–Walker (FLRW) metric yields the Lambda-CDM model, widely recognized as the "standard model of cosmology" due to its remarkable consistency with empirical observations.

By 2013, the Lambda-CDM model had demonstrated consistency with a progressively stringent array of cosmological observations, including data from the Planck spacecraft and the Supernova Legacy Survey (SNLS). Initial findings from the SNLS indicated that the average behavior, specifically the equation of state, of dark energy closely approximates Einstein's cosmological constant, with a precision of 10%. More recent data from the Hubble Space Telescope Higher-Z Team suggest that dark energy has been a persistent component for at least 9 billion years, encompassing the epoch preceding the universe's accelerated expansion.

In March 2025, the Dark Energy Spectroscopic Instrument (DESI) collaboration is scheduled to announce the discovery of evidence for evolving dark energy. This evidence stems from an analysis integrating DESI data on baryon acoustic oscillations (BAO) with cosmic microwave background (CMB), weak lensing, and supernovae datasets, exhibiting a statistical significance between 2.8 and 4.2σ. These findings imply a gradual decrease in the density of dark energy over time.

Observational Evidence

Although indirect, the evidence supporting the existence of dark energy originates from three distinct and independent observational domains:

• Measurements of cosmic distances and their correlation with redshift indicate that the universe's expansion has been more pronounced during its later stages compared to its earlier history.
• A theoretical imperative for an additional energy component, distinct from both ordinary matter and dark matter, is required to account for the observationally flat geometry of the universe, characterized by the absence of any detectable global curvature.
• Observations of large-scale wave patterns in the universe's mass density.

Supernovae Observations

In 1998, the High-Z Supernova Search Team presented observations of Type Ia ("one-A") supernovae. The Supernova Cosmology Project subsequently proposed in 1999 that the universe's expansion is accelerating. The 2011 Nobel Prize in Physics recognized the leadership of Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess in this discovery.

Since these initial findings, the observations have been substantiated by multiple independent lines of evidence. Data from cosmic microwave background analyses, gravitational lensing studies, investigations into the large-scale structure of the cosmos, and refined supernova measurements have all aligned with the Lambda-CDM model. A counterargument posits that the sole evidence for dark energy stems from observations of cosmic distances and their corresponding redshifts. Specifically, cosmic microwave background anisotropies and baryon acoustic oscillations merely indicate that distances at a particular redshift exceed those predicted by a "dusty" Friedmann–Lemaître universe, given the locally measured Hubble constant.

Supernovae are instrumental in cosmology due to their efficacy as standard candles across cosmological distances. They enable researchers to ascertain the universe's expansion history by examining the correlation between an object's distance and its redshift, which quantifies its recession velocity. This relationship approximates linearity, consistent with Hubble's law. While redshift measurement is comparatively straightforward, determining an object's precise distance presents a greater challenge. Astronomers typically employ standard candles—celestial objects with a known intrinsic brightness, or absolute magnitude. This knowledge facilitates the calculation of an object's distance based on its observed brightness, or apparent magnitude. Type Ia supernovae represent the most precise standard candles identified for cosmological distances, owing to their exceptional and uniform luminosity.

Recent supernova observations are consistent with a universe composed of 66.6% dark energy and 33.4% a composite of dark matter and baryonic matter, under the assumption of a flat Lambda-CDM cosmological model.

Large-scale structure

The theory of large-scale structure, which describes the formation of cosmic structures, including stars, quasars, galaxies, and galaxy clusters, further indicates that the universe's matter density constitutes merely 30% of the critical density.

In 2011, the WiggleZ galaxy survey, encompassing over 200,000 galaxies, furnished additional evidence supporting the existence of dark energy, despite its underlying physics remaining elusive. Conducted by the Australian Astronomical Observatory, the WiggleZ survey mapped these galaxies to ascertain their redshifts. By leveraging the characteristic voids of approximately 150 Mpc diameter, regularly imprinted by baryon acoustic oscillations and encircled by galaxies, these voids served as standard rulers. This enabled distance estimations for galaxies up to 2,000 Mpc (redshift 0.6), thereby facilitating precise calculations of galactic speeds from their redshift and distance. The collected data corroborated cosmic acceleration extending back to half the universe's current age (approximately 7 billion years) and constrained its inhomogeneity to one part in ten. This offers an independent confirmation of cosmic acceleration, distinct from supernova observations.

Cosmic microwave background

The presence of dark energy, irrespective of its specific manifestation, is essential to reconcile the observed spatial geometry with the universe's total matter content. Analyses of cosmic microwave background anisotropies suggest that the universe approximates a flat geometry. A flat universe necessitates that its total mass-energy density precisely matches the critical density. However, the aggregate matter content of the universe (comprising both baryonic and dark matter), derived from the cosmic microwave background spectrum, constitutes only approximately 30% of this critical density. This discrepancy implies the necessity of an additional energy component to account for the remaining 70%. A seven-year analysis by the Wilkinson Microwave Anisotropy Probe (WMAP) spacecraft estimated the universe's composition as 72.8% dark energy, 22.7% dark matter, and 4.5% ordinary matter. Subsequent research in 2013, utilizing observations from the Planck spacecraft's cosmic microwave background data, provided a refined estimate: 68.3% dark energy, 26.8% dark matter, and 4.9% ordinary matter.

Late-time integrated Sachs–Wolfe effect

The accelerated expansion of the cosmos induces the flattening of gravitational potential wells and hills during the passage of photons, resulting in the formation of cold and hot spots within the cosmic microwave background, which correlate with extensive supervoids and superclusters. This phenomenon, known as the late-time Integrated Sachs–Wolfe (ISW) effect, serves as a direct indicator of dark energy's presence in a flat universe. Its significant detection was documented in 2008 by Ho et al. and Giannantonio et al.

Hubble Constant Observational Data

A novel approach for testing the evidence of dark energy, employing observational Hubble constant data (OHD)—also known as cosmic chronometers—has attracted significant attention recently.

The Hubble constant, denoted as H(z), is determined as a function of cosmological redshift. Observational Hubble constant data (OHD) directly monitors the universe's expansion history by employing passively evolving early-type galaxies as "cosmic chronometers." Consequently, this method establishes standard clocks within the cosmos. The fundamental principle involves measuring the differential age evolution of these cosmic chronometers as a function of redshift, thereby yielding a direct estimation of the Hubble parameter.

H ( z ) = −<!-- − --> §2122§ §2425§ + z d z d t ≈ − §5758§ §6061§ + z Δ z Δ t . {\displaystyle H(z)=-{\frac {1}{1+z}}{\frac {dz}{dt}}\approx -{\frac {1}{1+z}}{\frac {\Delta z}{\Delta t}}.}

The utilization of a differential quantity, specifically ⁠Δz/Δt⁠, offers enhanced information and computational advantages, as it can mitigate numerous common issues and systematic effects. While analyses involving supernovae and baryon acoustic oscillations (BAO) rely on integrals of the Hubble parameter, the quantity ⁠Δz/Δt⁠ provides a direct measurement. Consequently, this methodology has been extensively applied to investigate accelerated cosmic expansion and to characterize the properties of dark energy.

Theories of Dark Energy

Given its classification as a hypothetical force with indeterminate characteristics, dark energy remains a prominent subject of ongoing research. This complex problem is approached from multiple perspectives, including the modification of the established theory of gravity (general relativity), efforts to precisely define dark energy's properties, and the exploration of alternative explanations for observed cosmological data.

The Cosmological Constant

The most straightforward hypothesis for dark energy posits it as an intrinsic, fundamental energy inherent to space itself. This concept is embodied by the cosmological constant, typically symbolized by the Greek letter Λ (Lambda), which gives rise to the Lambda-CDM model. In accordance with the mass-energy equivalence principle, E = mc§89§, Einstein's theory of general relativity predicts that this intrinsic energy exerts a gravitational influence. It is occasionally referred to as vacuum energy due to its representation of the energy density of empty space, or vacuum.

A significant unresolved issue arises from quantum field theories, which predict a cosmological constant that is approximately 120 orders of magnitude larger than observed. This discrepancy would necessitate an almost, but not perfectly, precise cancellation by an equally substantial term of opposing sign.

Some supersymmetric theories necessitate a cosmological constant that is precisely zero. The consistency of a positive cosmological constant with straightforward interpretations of string theory remains uncertain, particularly in scenarios where the universe is posited as a false vacuum characterized by a positive cosmological constant. Ulf Danielsson et al. have conjectured the non-existence of such a state. Nevertheless, should string theory preclude such a false vacuum, alternative dark energy models, including quintessence, might still be considered viable.

Quintessence

Within quintessence models of dark energy, the observed acceleration of the universe's scale factor is attributed to the potential energy of a dynamic field, known as the quintessence field. A key distinction between quintessence and the cosmological constant is quintessence's capacity for spatio-temporal variation. To prevent gravitational clumping and structure formation akin to matter, the field must possess an extremely low mass, resulting in a substantial Compton wavelength. In its most straightforward formulations, the quintessence field incorporates a canonical kinetic term, exhibits minimal coupling to gravity, and lacks higher-order operators within its Lagrangian.

Currently, no definitive evidence for quintessence exists, nor has its existence been disproven. Typically, quintessence models forecast a marginally slower acceleration in the universe's expansion compared to the cosmological constant. Certain researchers propose that compelling evidence for quintessence would manifest as violations of Einstein's equivalence principle or as spatio-temporal variations in fundamental constants. While scalar fields are predicted by both the Standard Model of particle physics and string theory, an issue analogous to the cosmological constant problem (or the challenge of constructing cosmological inflation models) arises: renormalization theory suggests that scalar fields ought to acquire substantial masses.

The coincidence problem addresses the question of why the universe's acceleration commenced at its observed epoch. Had acceleration initiated earlier, cosmic structures like galaxies would not have had sufficient time to form, precluding the emergence of life as currently understood. Advocates of the anthropic principle interpret this observation as corroboration for their hypotheses. Nevertheless, numerous quintessence models incorporate a "tracker" behavior, which offers a resolution to this problem. Within these models, the quintessence field's density closely follows (though remains lower than) the radiation density until the epoch of matter–radiation equality, at which point quintessence begins to act as dark energy, ultimately becoming the dominant component of the universe. This mechanism inherently establishes the low energy scale characteristic of dark energy.

In 2004, analyses correlating dark energy evolution with cosmological data indicated that the equation of state ${\displaystyle \omega }$ might have traversed the cosmological constant boundary ω<!-- ω --> = −<!-- − --> §3132§ {\displaystyle \omega =-1} from a value greater than to one less than -1. A no-go theorem has demonstrated that this scenario necessitates models incorporating a minimum of two distinct scalar field types. This particular scenario, termed Quintom, was initially proposed by Xinmin Zhang's research group in 2004.

Specific variants of quintessence include phantom dark energy, characterized by an energy density that increases over time, and k-essence (kinetic quintessence), which features a non-standard kinetic energy form, potentially including negative kinetic energy. These variants can exhibit anomalous properties; for instance, phantom dark energy is theorized to lead to a 'Big Rip' singularity.

In 2021, a research collective posited that observations related to the Hubble tension might suggest the viability solely of quintessence models possessing a non-zero coupling constant.

Interacting dark energy

This class of theories endeavors to formulate a unified theoretical framework for both dark matter and dark energy, conceptualizing them as a singular phenomenon that modifies gravitational laws across various scales. For instance, such theories might treat dark energy and dark matter as distinct manifestations of the same unknown substance, or propose the decay of cold dark matter into dark energy. Another category of theories unifying dark matter and dark energy comprises covariant modified gravity theories. These frameworks alter spacetime dynamics such that the modified dynamics account for phenomena attributed to the presence of dark energy and dark matter. In principle, dark energy could interact not only with the remainder of the dark sector but also with ordinary matter. However, cosmology alone is insufficient for robustly constraining the strength of the coupling between dark energy and baryons, necessitating the employment of other indirect techniques or laboratory searches. In the early 2020s, it was temporarily hypothesized that an anomalous signal detected in the XENON1T detector in Italy might have originated from a chameleon model of dark energy, but subsequent experiments refuted this possibility.

Models of Variable Dark Energy

The density of dark energy may have fluctuated over the universe's history. Contemporary observational data enables the estimation of dark energy's current density. Baryon acoustic oscillations facilitate the investigation of dark energy's influence throughout cosmic history and the constraint of parameters governing its equation of state. Consequently, numerous models have been advanced. A prominent example is the Chevallier–Polarski–Linder (CPL) model. Additional notable models include Barboza & Alcaniz (2008), Jassal et al. (2005), Wetterich (2004), and Oztas et al. (2018).

Emerging observational evidence suggests that dark energy exhibits a temporal decline. Data from the Dark Energy Spectroscopic Instrument (DESI), which monitors the scale of baryon acoustic oscillations throughout cosmic expansion, indicates that the dark energy density was approximately 10% higher 4.5 billion years ago. Nevertheless, insufficient data currently exists to definitively preclude dark energy from being a cosmological constant.

Alternatives to Dark Energy

Modified Gravity Theories

The evidence supporting dark energy is fundamentally predicated upon the theory of general relativity. Therefore, it is plausible that alterations to general relativity could obviate the necessity for dark energy. Numerous such theoretical frameworks exist, and active research continues. The measurement of the speed of gravity during the first gravitational wave event detected by non-gravitational means (GW170817) invalidated a substantial number of modified gravity theories as viable explanations for dark energy.

Astrophysicist Ethan Siegel asserts that, despite the mainstream media attention garnered by such alternatives, the overwhelming consensus among professional astrophysicists affirms the existence of dark energy, and no extant competing theories account for observational data with the same degree of precision as the standard dark energy model.

Observational Skepticism

Some alternatives to dark energy, such as inhomogeneous cosmology, endeavor to elucidate observational data through a more sophisticated application of established theoretical frameworks. In this scenario, dark energy is posited not to exist, but rather to represent an artifact of measurement. For example, if observers are located in an underdense region of space, the observed cosmic expansion rate might be erroneously interpreted as a temporal variation or acceleration. Another methodology employs a cosmological extension of the equivalence principle to demonstrate how space could manifest accelerated expansion within the voids adjacent to our local galaxy cluster. Although individually subtle, the cumulative impact of such effects over cosmological timescales could become substantial, thereby generating the perception of cosmic acceleration and suggesting residence within a "Hubble bubble." Further hypotheses propose that the universe's accelerated expansion is an illusory phenomenon resulting from our relative motion with respect to the cosmic rest frame, or that the statistical methodologies utilized were inherently flawed. Direct laboratory detection experiments have thus far failed to identify any force attributable to dark energy.

Explanations of dark energy based on observational skepticism have generally received limited acceptance within the cosmological community. For instance, a study proposing that the anisotropy of the local universe was misinterpreted as dark energy was promptly refuted by a subsequent publication identifying errors in the initial work. Similarly, another investigation challenging the fundamental premise that Type Ia supernova luminosity remains constant irrespective of stellar population age was rapidly rebutted by other cosmologists.

Black Holes as a General Relativistic Phenomenon

The theory of "cosmological coupling" has been advanced as an alternative explanation for dark energy phenomena. This hypothesis posits that the local description of black holes, governed by the Kerr metric, must align with the global cosmic description provided by the Friedmann-Robertson-Walker metric, rather than assuming spatially flat spacetime at infinite distances. Consequently, black holes are predicted to accrue mass as the universe undergoes expansion. This accretion rate is observed to be proportional to a§67§ , where a represents a scale factor for matter within the cosmos. Such a rate implies that the energy density associated with black holes remains constant over cosmic time, thereby emulating the behavior of dark energy. However, other astrophysicists express skepticism, citing various publications that contend the theory is inconsistent with additional observational data. Conversely, some research indicates that current observations are insufficient to definitively exclude this model.

Shockwave Cosmology

Shockwave cosmology, introduced by Joel Smoller and Blake Temple in 2003, conceptualizes the Big Bang as an explosion occurring within a black hole, thereby generating the expanding volume of space and matter that constitutes the observable universe. A related hypothesis, developed by Smoller, Temple, and Vogler, suggests that this shockwave could have led to our region of the universe possessing a lower density than its surroundings, thus accounting for the accelerated expansion typically ascribed to dark energy. They further propose a testable prediction for this alternative theory: a universe governed by dark energy would yield a cubic correction to redshift versus luminosity of C = −0.180 at a = a, whereas the Smoller, Temple, and Vogler model predicts a positive value for C. Their more precise calculation for the shockwave model alternative indicates that the cubic correction to redshift versus luminosity at a = a is C = 0.359.

While shockwave cosmology generates a universe that appears "essentially identical to the aftermath of the Big Bang," cosmologists generally agree that it requires substantial further development before it can be regarded as a more advantageous model than the standard Big Bang theory for explaining the cosmos. Specifically, and particularly concerning its proposed alternative to dark energy, the model must account for Big Bang nucleosynthesis, the quantitative characteristics of cosmic microwave background anisotropies, the Lyman-alpha forest, and observational data from galaxy surveys.

Implications for the Universe's Ultimate Fate

Cosmologists estimate that cosmic acceleration commenced approximately 5 billion years ago. Prior to this period, the universe's expansion is believed to have been decelerating, primarily due to the gravitational attraction of matter. In an expanding universe, the density of dark matter diminishes more rapidly than that of dark energy, leading to the eventual dominance of dark energy. For instance, a doubling of the universe's volume results in a halving of dark matter density, whereas the density of dark energy remains nearly constant (or precisely constant in the scenario of a cosmological constant).

Future projections vary significantly depending on the specific models of dark energy employed. In scenarios involving a cosmological constant, or any model positing indefinite acceleration, galaxies beyond the Local Group would exhibit a line-of-sight velocity that progressively increases over time, ultimately surpassing the speed of light. This phenomenon does not contravene special relativity, as the "velocity" concept applied in this context differs from velocity within a local inertial frame, where massive objects remain constrained to sub-light speeds. (The intricacies of defining relative velocity in cosmology are further explored in discussions concerning the proper distance.) Furthermore, due to the temporal decrease of the Hubble parameter, it is conceivable for a galaxy receding faster than light to nonetheless emit a signal that eventually reaches an observer.

Nevertheless, the accelerating expansion is predicted to cause most galaxies to eventually traverse a cosmological event horizon. Beyond this boundary, any emitted light will be perpetually unable to reach us, as its "peculiar velocity" towards us will never overcome the expansion velocity away from us. (These distinct velocity concepts are also elaborated upon in discussions of the proper distance.) Assuming a constant dark energy, such as a cosmological constant, the present distance to this cosmological event horizon is approximately 16 billion light-years. Consequently, a signal from an event occurring at present would eventually reach us if the event originated within 16 billion light-years, but it would never arrive if the event occurred beyond that distance.

As galaxies approach this cosmological event horizon, their emitted light will undergo progressively greater redshift, eventually reaching wavelengths too extensive for practical detection, causing these galaxies to effectively disappear from observation (see Future of an expanding universe). In this scenario, Planet Earth, the Milky Way, and the entire Local Group of galaxies, including the Milky Way, would remain largely unaffected while the broader universe recedes and becomes unobservable. Ultimately, the Local Group itself would experience heat death, a fate previously theorized for a flat, matter-dominated universe prior to the discovery of cosmic acceleration.

Beyond these projections, more speculative theories exist regarding the universe's ultimate destiny. The phantom dark energy model, for instance, predicts divergent expansion, suggesting that the effective force of dark energy would continuously intensify until it overwhelms all other fundamental forces. This scenario culminates in a "Big Rip," where dark energy would progressively dismantle all gravitationally bound structures, from galaxies to solar systems, and eventually overcome electrical and nuclear forces, tearing apart atoms themselves. Conversely, dark energy might diminish over time or even become attractive. These uncertainties allow for the possibility that gravity could eventually dominate, leading to a "Big Crunch" where the universe contracts upon itself. Another hypothesis suggests a dark energy cycle, implying a cyclic universe model where each iteration (from a Big Bang to an eventual Big Crunch) spans approximately a trillion (10¹²) years. Although observational data do not currently support these alternative scenarios, they have not been definitively disproven.

In philosophy of science

Astrophysicist David Merritt characterizes dark energy as an "auxiliary hypothesis," an ad hoc postulate introduced into a theory to accommodate falsifying observations. He contends that the dark energy hypothesis functions as a conventionalist hypothesis, meaning it lacks additional empirical content and is therefore unfalsifiable according to Karl Popper's definition. Nevertheless, this perspective is not universally accepted within the scientific community.

Notes

Notes

References

The Euclid ESA Satellite is a mission dedicated to mapping the geometry of the dark universe.

• Euclid ESA Satellite, a mission to map the geometry of the dark universe
• "Surveying the dark side" by Roberto Trotta and Richard Bower, published in Astron.Geophys..