Question

What evidence do we have that there is dark matter in spiral galaxies?

Short answer

The constant rotational speeds of stars in spiral galaxies, despite the expected decline, suggest the existence of invisible dark matter providing extra gravitational force.

Step-by-step solution

Understanding Spiral Galaxies

Spiral galaxies are characterized by their flat, rotating disks, central bulges, and spiral arms. These galaxies contain stars, gas, and dust. Understanding their rotational dynamics is crucial to explore the presence of dark matter.

Observing Rotational Curves

Astronomers measure the rotational curves of spiral galaxies by observing the rotational speed of stars at different distances from the galactic center. According to Newtonian physics and visible mass, the rotational speed should decrease with distance.

Anomalous Rotation Speeds

When observing spiral galaxies, it was found that the rotational speeds of stars remained nearly constant out to large distances from the center, rather than decreasing. This observation contradicts the expected decline in speed.

Hypothesis of Dark Matter

The consistent rotational speed suggests the presence of additional unseen mass. This led scientists to hypothesize the existence of dark matter, an unseen substance that exerts gravitational forces, affecting these speeds.

Supporting Evidence from Other Sources

Further support for dark matter comes from various astrophysical observations, such as gravitational lensing and cosmic microwave background measurements, reinforcing its role in galaxy dynamics.

Key concepts

Spiral Galaxies

Spiral galaxies are among the most captivating structures in the universe. Imagine a spinning whirlpool in the vastness of space, which is essentially what these galaxies are like, with their distinguishing flat, rotating disks.
They have bright spiral arms that wind outward from a central point, known as the galactic bulge. This central region is densely packed with stars, setting a striking contrast against the spiral arms where newer stars often form.
Spiral galaxies consist of not only stars but also large amounts of dust and gas which play a critical role in star formation. The dynamic motion of these components underpins the gravitational forces that define the galaxy’s structure.
Understanding these dynamics is crucial, as the movement within spiral galaxies can provide clues about unseen forces, like dark matter, that influence their behavior.

Rotational Curves

To study the movement in spiral galaxies, astronomers often construct rotational curves. This involves measuring how fast stars and other matter orbit around the center of the galaxy.
Rotational curves are fundamental because they can reveal discrepancies in our understanding of visible and invisible forces within galaxies. According to classic Newtonian physics, the rotational speed of stars should decrease as we move away from the central bulge of the galaxy.
However, when observational data is plotted on a rotational curve, it displays a rise to a certain point, and then a flat, constant speed, defying initial expectations based on the observable mass alone. This peculiar flatness suggests that additional unseen mass exerts gravitational forces impacting these speeds.

Anomalous Rotation Speeds

One of the most intriguing phenomena in spiral galaxies is their anomalous rotation speeds. Astronomers expected that stars far from the center would orbit more slowly, similar to how planets farther from the sun move at slower speeds.
However, observations have shown that stars in the outer regions of spiral galaxies are rotating much faster than expected, at nearly the same speed as those nearer to the center.
This continuous speed at great distances creates a puzzle, as it cannot be explained by the gravitational forces from the visible mass alone. This anomaly in rotation speeds is one of the strongest indicators pointing toward the existence of a mysterious and invisible substance in the universe.

Newtonian Physics

Newtonian physics provides the foundation for our understanding of gravitational forces within galaxies. According to Isaac Newton's laws, gravitational attraction decreases with distance.
This means that celestial bodies, like stars, should orbit more slowly as they move away from the galactic center, due to lesser gravitational influence from the central mass.
Thus, the observed flatness in the rotational curves contradicts this classical view, suggesting that Newtonian physics alone cannot fully account for the dynamics in spiral galaxies.
The implication is that there must be another non-visible form of matter exerting gravitational influence, which has led scientists to entertain the existence of dark matter within these galaxies.

Gravitational Lensing

Gravitational lensing is a fascinating concept that further supports the presence of dark matter in spiral galaxies. It occurs when a massive object, like a galaxy, bends the light from a more distant object behind it, acting like a lens.
This effect is predicted by Einstein's theory of general relativity, showing that mass can warp the fabric of space-time, impacting light paths.
By studying gravitational lensing, astronomers can estimate the total mass of a galaxy, much of which cannot be seen directly. Frequently, these measurements reveal more mass than what is visible—suggesting the presence of dark matter.

• This phenomenon gives credence to the hypothesis that unseen forces contribute significantly to galactic dynamics.
• Gravitational lensing helps map dark matter, offering insights into its distribution and proportion within galaxies.

In essence, gravitational lensing not only illuminates the presence of dark matter but also aids in understanding the structure and evolution of the universe.