Gravitational Instability Theory Explained


How do structures form in the universe?

The gravitational instability theory recognizes that material can only be brought together to form structures with the use of gravity. Gravity is the only long-range force that can provide such an influence on large scales.

This means the large Jovian planets would be formed directly from the gaseous instabilities and the gravity of those stabilities. When the gravitational instability is strong enough, it can alter the structure of the disk and then break it up into bound objects. This allows the planet to form through the fragments as the gaseous instability collapses, creating the gravity required to bind the fragments back together again.

There are two factors that must be in place for a gravitational instability to occur.

1. Mass. The disk must be large enough for gravity to become unstable in the first place.
2. Temperature. When disks are colder, then they are more likely to become unstable.

Then the thermodynamics of the disk must also have certain features to determine the likelihood of fragmentation occurring. There must be heating actions that occur with the temperature to encourage fragmentation. Heating may be caused by external radiation, internal heat, or turbulence.

Where Can Gravitational Instability Occur?

Gravitational instability can occur in any region of space that become sufficiently cool enough. It will also form in areas of space that develop a high enough surface density. This will cause the gases in that region of space to produce specific effects.

It most often forms local or global spiral waves. It may also produce self-gravitating turbulence. There could be mass or angular momentum transported through the torque of long-range gravity influences. In extreme cooling situations, fragmentation into clumps may occur, which is how we get the potential to form a giant planet.

In this type of situation, when cooling is extreme, the planets have the possibility to form very quickly. Simulations have shown that about 1,000 years may be all that is required to create the basic formative structures of a planet under gravitational instability.

All of this requires that the gas disk in question, placed under pressure, creates its own self-gravity because of its overall mass over time. This self-gravity, when influenced by the surrounding long-range gravity, begins to place a compressive force upon the core of the disk in question. With enough pressure, the disk can begin to fragment.

This fragmentation then leads to the development of a core, which then increases the amount of self-gravity that can be found at that location. The increased gravity continues the fragmentation process, allowing a Jovian planet to begin forming over time.

Relaxed Disks vs Unrelaxed Disks in Gravitational Instability Theory

How a planet can form through the gravitational instability theory depends on the gas disk itself. If it is an isolated relaxed disk, the it may require more than 10% of a host star’s mass in order for the planet to begin forming. Rapid cooling and the rapid loss of angular momentum must also occur. Radiative cooling would likely be inefficient because of the high opacity in the surface density of the disk in question.

Unrelaxed disks need less mass in order to form a planetary body because they have their own mass that they can call upon to create a collapse. As this process occurs, it will begin to relax toward the achievement of an equilibrium. Even a lower-mass star instead of a planet could potentially form from the gravitational instability of an unrelaxed disk.

Disks are also believed to be able to interact with one another to form heavenly bodies as well. Large disks with frequent interactions can share materials with one another and have gravity affect them in different ways. Should that gravitational influence result in a collapse, then shared materials through fragmentation between the two disks can form one body formed from the two different influences.

Did Our Universe Come from Gravitational Instability?

From our observations of the galaxies as they move away from us, we can understand that in the past, these galaxies were closer together. This means the universe has changed over time from its initial creation, becoming less dense and cooler.

We can replicate the beginning moments of the universe using large particle accelerators, but we do not have the means to understand the earliest states of the universe upon its initial formation.

This means the process of gravitational instability could have been an influence on the formation of the universe itself. With a gas disk becoming fragmented on a large enough scale, the resulting movement outward would make the universe similar to a very large “planet” in which all known reality exists.

Many of the atoms that were produced by what we understand from the creation of our universe were hydrogen and helium. These atoms formed into clouds, which would eventually form into stars and planets based on, in part, the principles of gravitational instability.

Dark Energy and the Gravitational Instability Theory

We look at the long-range force of gravity as an influence for planetary formation, but there is also evidence to suggest that dark energy dominates throughout all of our known space. More than 70% of the total energy density of the universe could be dark energy. It is this energy that is the currently accepted explanation as to why the universe is continue to expand.

One proposed form of dark energy is called the “cosmological constant.” This form of energy density fills the vacuum of space and essentially “holds back” gravity. The idea was abandoned because of observations that the galaxies are moving away from each other, but we also know that movement can counteract gravity. This is why we can launch rockets into space. With enough force, you can overcome gravity.

So this dark energy could also be a long-range influence on the formation of planets and stars because it could potentially keep the self-developing gravity of disk in place, allowing a core to form.

How Can Gravitational Instability Occur in an Expanding Universe?

The formation of Jovian planets seems to contradict the idea that the universe is expanding. After all, to form these planets, the gases must essentially collapse upon themselves, retracting instead of expanding.

We must remember that there are other forces in play beyond the expansion of the universe. The law of entropy, for example, shows that aging occurs within the universe, even though it continues to expand. Over time, this aging process can cause stars to explode or compress upon themselves as well if the gas formulations within the star change over time.

When a star explodes, it sends out shockwaves of turbulence. This turbulence occurs at a very high compression rate, including the interacting shocks. As this wave of turbulence expands outward throughout a local region or sector, if large enough, it can have an effect on the gas disks that are present. When gravity is added to the mix, the instability is enough to from the core of what can become a planet.

Potential Problems with the Gravitational Instability Theory

The primary issue with the gravitational instability theory is the timing. How efficient the cooling process happens to be for convective or radiative cooling can be predicted, but it has not been directly observed, when considering the formation of planets. If there is less efficiency in this process, then the entire system breaks down.

There are also considerations to the gaseous disk mass that would be required for this process to work in nature. A high disk mass would almost always be required, using up to 10% of the host star’s resources in the process. This is about 10 times more than the disk masses that have been directly observed. Disks would need to be infinitesimally thin in order for the process to work effectively.

It would also call into question the formation of certain planets in our own solar system, such as Jupiter or Saturn, and what that would mean for the formation of our own planet in a temperate zone.

The fact of the matter is that we understand very little about what makes the universe tick right now. We can speculate and predict, using computer models and simulations, what may cause planets and stars to form, but that is all we can do at current fundamental levels. Gravity may have an influence, but that influence could be affected by anti-gravity and the energy of dark matter – or it may all work together in some unknown way.

What we do know is this. Distance measurements and their relationship to redshift suggest the universe is expanding more than it did in its early life. The natural cooling process seems to suggest that the ideas of the gravitational instability theory could provide one explanation as to the current structure of our universe. It may be just one method of planet formation or one of several methods that can be used.

As we gain more observational data from sources such as the Hubble Telescope, we will be able to refine what we know about the universe. Until then, this theory provides one potential method of understanding.