Ar tamsioji materija – tik visatos gravitacija pačiai sau?

Or dark matter – just the universe's gravity for itself?

Linas Juozenas

What if dark matter is merely the mutual gravitational attraction of the entire Universe?

A thorough investigation of this intriguing idea

Dark matter is one of the greatest mysteries of modern cosmology and astrophysics. Observations, including galaxy rotation curves, gravitational lensing, and large-scale structure formation, indicate that there is a form of matter in the Universe that does not interact with light—hence called "dark." Based on Newton's and Einstein's concepts of gravity, visible, "ordinary" matter (protons, neutrons, electrons) makes up only about 5% of the total energy and matter content of the Universe, while dark matter accounts for about 27% (the remaining portion is dark energy).

But what if this missing mass doesn't exist at all? Perhaps it is just the mutual gravitational effect of the Universe itself: the small gravitational contributions of all stars, planets, and gas particles, which together create phenomena we interpret as "dark matter." This is an intriguing idea: could we abandon the concept of dark matter as a separate component and explain everything solely by the combined gravitational influence of visible matter on a massive scale?

In this article, we will thoroughly explore this idea—we will review the evidence for the existence of dark matter, scientific attempts to explain this phenomenon, and why the notion "it's just gravity from everything that exists" is both appealing and, unfortunately, insufficient when looking at detailed observational data.

1. Evidence for the existence of dark matter

1.1 Galaxy rotation curves

One of the first clear pieces of evidence for the existence of dark matter is the measurement of the orbital speeds of stars at the edges of galaxies. According to Newtonian mechanics, the orbital speed of stars at the galaxy's edge should decrease with increasing distance from the center—similar to how the speed of planets in our Solar System decreases as they move away from the Sun.

However, astronomers have observed that stars in the outermost regions of spiral galaxies move much faster than predicted by conventional calculations. This phenomenon, called "flat rotation curves," indicates that there is much more mass than we can determine from electromagnetic radiation (light at various wavelengths). If only visible matter (stars, gas, dust) existed in the galaxy, the orbits of distant stars would be slower. So the simplest explanation is that there is an additional layer of invisible mass, i.e., dark matter.

1.2 Gravitational lensing

Gravitational lensing is the ability of massive objects to bend light, as described in Einstein's general theory of relativity. Observing galaxy clusters, it is seen that their effect on the images of more distant galaxies (lensing) is much stronger than can be explained by visible matter alone. To explain this effect, additional mass is required – again pointing to dark matter.

A famous example is the so-called Bullet Cluster collision, where two galaxy clusters passed through each other. Hot gas (visible in the X-ray range) was slowed down due to interactions, while the strongest gravitational influence moved ahead. This suggests that part of the mass interacts almost not at all electromagnetically (i.e., does not get caught like ordinary gas), but has a significant gravitational effect.

1.3 Cosmological observations and structure formation

Looking at the cosmic microwave background radiation (CMB) – the "afterglow" of the Big Bang, scientists observe density irregularities. These irregularities eventually grew into the galaxies and clusters we see today. Computer simulations of the Universe's structure formation show that without dark matter, the development of such density "seeds" to their current sizes would be extremely difficult to explain or even impossible. Without dark matter, the formation of a highly uneven matter structure (galaxies, galaxy clusters) from the nearly homogeneous early Universe would be too slow.

2. Proposed idea: the collective gravitational attraction of all matter

The idea "maybe dark matter is just the mutual gravitational attraction of everything that exists" seems attractive at first glance. After all, gravity acts over unlimited distances; no matter how far apart two objects in the Universe are, they still attract each other. If we imagine an innumerable number of stars and galaxies, perhaps their combined gravitational effect could explain that extra mass.

2.1 Intuitive appeal

1. Unified explanation of gravity: In part, this seems like a unifying idea. Instead of introducing a new type of matter, we could claim that we are only observing the collective effect of matter known to us.
2. Simplicity: Many find it appealing to believe that only baryonic (ordinary) matter exists and nothing more. Perhaps until now, we have simply underestimated the overall gravity of all this matter, especially on large scales.

However, this hypothesis faces serious challenges when applied to precise observational data and well-tested physical theories. Let's look at where the problems arise.

3. Why the mutual gravity of known matter alone is insufficient

3.1 Conventional versus modified gravity

Attempts to explain cosmic phenomena without dark matter often fall into the realm of "modified gravity" theories. Instead of introducing a new type of matter, it is proposed to adjust the laws of gravity on the scale of the Universe. One of the most famous examples is MOND (Modified Newtonian Dynamics). MOND claims that in regions of extremely low acceleration (such as at the edges of galaxies), gravity behaves differently than predicted by Newton or Einstein.

If the general gravity of all the Universe's matter were the force mistakenly called dark matter, it would essentially have to act as a kind of modified gravity version. Proponents of MOND and similar theories try to explain galaxy rotation curves and other phenomena. However, MOND, while it may fit some observations (for example, galaxy rotation curves), struggles to align with other facts (for example, Bullet Cluster gravitational lensing data).

Therefore, any theory claiming that "dark matter" is caused solely by the general gravity of ordinary matter would have to successfully explain not only galaxy rotation curves but also lensing, cluster collisions, and large-scale structure formation. So far, no alternative theory has fully replaced the dark matter hypothesis to match all observations.

3.2 The inverse square law and cosmic scales

The gravitational force weakens with the square of the distance (according to Newton's law of universal gravitation). On cosmic scales, there is a real, though small, attraction of distant galaxies, clusters, and filaments, but this force decreases quite rapidly with distance. Observational data show that visible (baryonic) matter alone is insufficient and is not distributed in a way that would create the gravitational effects attributed to dark matter.

If we tried to add up all the visible matter in the Universe and calculate its gravitational effect on various cosmic scales, it would turn out that we still cannot reproduce the actual rotation curves of galaxies, lensing effects, or the rate of structure formation. Simply put, in a Universe containing only baryonic matter, the gravitational force would be too weak to explain the observed effect.

3.3 Bullet Cluster and the distribution of "missing" mass

Bullet Cluster is a particularly striking example. When two galaxy clusters collide, the ordinary matter (mostly hot gas) was slowed down due to interaction, while the other – almost non-interacting – mass component (believed to be dark matter) successfully passed through the collision without slowing down. Gravitational lensing data show that most of the mass "moved away" further, lagging behind the glowing gas.

If the missing mass were explained simply by all the matter in the Universe, one would expect the mass distribution to align more closely with visible matter (slowed gas). However, the observed discrepancy between visible gas and gravitationally active mass indicates the existence of additional matter that does not interact electromagnetically—dark matter.

4. "Gravity of all matter" and cosmology

4.1 Constraints from Big Bang Nucleosynthesis

The lightest chemical elements—hydrogen, helium, and a bit of lithium—formed in the early Universe. This process is called Big Bang Nucleosynthesis (BBN). The abundance of light elements is sensitively dependent on the total density of baryonic (ordinary) matter. Observations of the cosmic microwave background and studies of these elemental ratios show that there cannot be too much baryonic matter in the Universe—otherwise, it would contradict the observed amounts of helium or deuterium. In short, BBN indicates that ordinary matter makes up about 5% of the Universe's energy and matter budget.

4.2 Measurements of the Cosmic Microwave Background Radiation

High-resolution data obtained from satellites such as COBE, WMAP, and Planck have allowed cosmologists to measure CMB temperature fluctuations with extraordinary precision. The nature of these fluctuations, especially their angular power spectrum, enables the estimation of the density of various components (dark matter, dark energy, and baryonic matter). These measurements align very well with the cosmological model in which dark matter is a separate, non-baryonic component. If the gravitational effect currently attributed to dark matter were merely the combined attraction of visible matter, the CMB power spectrum would look completely different.

5. Is there another way to say that dark matter is just "gravity"?

The idea "what if dark matter is actually just an imperfection in the laws of gravity?" has inspired various modified gravity theories. They propose to correct Einstein's General Relativity or Newtonian dynamics on galactic and larger scales, sometimes offering quite complex mathematical foundations. Such theories attempt to explain galaxy rotation curves and cluster lensing without additional, invisible particles.

Main challenges for modified gravity theories:

Adjustment: Gravity needs to be modified on the galactic scale, while simultaneously remaining consistent with Solar System observations and the general theory of relativity, which has been confirmed with high precision by numerous experiments.
Structure formation: Theories must explain not only the rotation curves of galaxies but also the formation of cosmic structures from the early times to the present day, matching observations across different epochs.
Relativistic effects: When changing the law of gravity, it is necessary not to contradict phenomena such as gravitational lensing or Bullet Cluster data.

Although “Lambda Cold Dark Matter” (ΛCDM) – the current standard cosmological model including both dark matter and dark energy (Λ) – has some shortcomings, so far no modified gravity theory has succeeded in explaining all observations as well as ΛCDM.

6. Conclusion

The idea that dark matter could simply be the mutual gravitational attraction of all matter in the Universe is interesting. It aligns with the pursuit of a simpler explanation that does not require a new, invisible matter concept. Essentially, this resonates with the old scientific and philosophical principle that Occam's razor advises eliminating unnecessary hypotheses.

But decades of astronomical and cosmological observations show that the amount of known matter alone does not explain the “missing mass” problem. Galaxy rotation curves, gravitational lensing data, large-scale structure formation rates, cosmic microwave background measurements, and Big Bang nucleosynthesis constraints – all suggest that there exists a type of matter that is without and in addition to the ordinary matter we know. Moreover, the Bullet Cluster and similar observations show that the invisible mass behaves differently from ordinary matter (for example, participating weakly in other, non-gravitational interactions).

Cosmology is, however, a constantly evolving scientific field. New observations – from gravitational waves to more precise galaxy distribution maps and even better CMB analysis – continuously improve our understanding. So far, most observational data indicate that dark matter indeed exists as a separate, non-baryonic type of matter. Yet open-mindedness and attentiveness to unexpected data remain very important – after all, science advances when hypotheses are tested and changed if they do not match new facts.

Currently, observations mostly support the idea that dark matter is a real, physical component. However, asking "could there still be an alternative?" is to uphold the spirit of scientific curiosity, which is especially needed to understand the mysteries of the Universe.

Item placed in your cart