Dark Matter and Dark Energy

# Unveiling the Mysterious Dark Side of Our Universe: An Expert Guide to Understanding Dark Matter and Dark Energy

Our magnificent universe hides many secrets beyond what our eyes can see and technology can detect. The normal matter that makes up galaxies, stars, and planets comprises only about 5% of the cosmos. The rest is dark matter and dark energy – invisible substances that govern the universe’s behavior on the grandest scales. Read on for an extensive primer on these dark cosmic puzzles that have scientists racking their brains.

Illuminating the Nature of Dark Matter

Dark matter is aptly named – it neither absorbs, emits, nor reflects light. Its existence is only inferred from gravitational effects on visible matter. So what is this transparent matter that permeates galaxies and the universe?

Galactic Rotation Reveals Dark Matter

Our first clues to dark matter came from the rotation of galaxies. Based on the visible mass, galaxies should spin more slowly. However, galaxies rotate faster than predicted. This suggests there is extra invisible matter providing extra gravity – hence the dark matter hypothesis was born.

Gravitational Lensing Confirms Dark Matter’s Presence

Another smoking gun for dark matter comes from gravitational lensing. Massive objects like galaxy clusters bend and distort the light from background galaxies and objects according to Einstein’s theory of general relativity. Mapping these gravitational lens effects enables astronomers to create a mass distribution – and they find way more mass than the visible components can account for. This provides independent confirmation that dark matter is real and not just theory.

The Cosmic Web Depends on Dark Matter

On even larger scales beyond individual galaxies, dark matter drives the formation of the cosmic web – the vast filamentary network of galaxies and galaxy clusters throughout the universe. Dark matter started clumping together early in cosmic history, creating gravitational wells that later attracted normal matter. Simulations show dark matter is the underlying cosmic glue determining the web’s structure.

Dark Matter Outweighs Regular Matter

Today, dark matter makes up about 85% of all matter in the universe. The remaining 15% is normal atomic matter that makes up observable galaxies, gas, stars, and planets. So the cosmos is dominated by a non-luminous, ethereal substance whose nature remains largely unknown.

WIMPs Lead the Dark Matter Candidate Race

The current frontrunners for what dark matter could be are WIMPs – weakly interacting massive particles. WIMPs only interact through gravity and possibly the weak nuclear force. They are thought to move slowly and be cold, meaning not moving at relativistic speeds. Many experiments are underway to try and directly detect WIMPs as they pass through Earth.

Alternative Candidates Include Axions and SIMPs

Though WIMPs are the most searched for dark matter candidates, they have so far eluded detection. This has opened the door to other possibilities like axions and SIMPs (strongly interacting massive particles). Axions are lighter particles that were proposed to solve some particle physics quandaries. SIMPs interact through the strong force rather than the weak force like WIMPs. The race is on to find the true identity of dark matter.

Dark Matter Formed in the Early Universe

According to the standard cosmological model, dark matter likely formed during the radiation-dominated era in the early universe, a few microseconds after the Big Bang. While normal matter was still ionized plasma, dark matter began collapsing into clumps and structures under its own gravity. This provided a sort of cosmic scaffolding for galaxies to later form and evolve.

Dark Matter Drives Galaxy and Structure Formation

After the universe became matter-dominated, normal atomic matter fell into the preexisting gravitational wells created by dark matter. This ultimately gave rise to the first stars, galaxies, and larger scale cosmic structures we observe in the universe today. Computer simulations suggest dark matter was vital for this structure formation in the universe and still determines the motion and evolution of galaxies.

Now that you have a solid base knowledge of dark matter, let’s move on to unpacking the other giant cosmological mystery – dark energy.

Shedding Light on the Nature of Dark Energy

Dark energy is even more peculiar than its counterpoint dark matter. It is theorized to be a mysterious repulsive force that is accelerating the expansion of the universe. Discoveries in cosmology have built up our picture of this dominant ingredient of our universe.

Supernova Measurements Reveal Accelerating Expansion

The discovery of dark energy can be traced to studies of Type Ia supernovae in the 1990s. Measuring distances to faraway supernovae showed that galaxies are receding from us at an accelerating rate, contrary to expectations of universal deceleration. This implies a previously unknown energy permeating all space and driving expansion.

Dark Energy Makes Up 68% of the Cosmos

Today, the density of dark energy is estimated to account for about 68% of the total energy content of the universe. When combined with dark matter at 27%, that leaves just 5% for ordinary matter that composes planets, stars, and interstellar gas. Our universe is dominated by these two shadowy substances.

Vacuum Energy is the Leading Explanation

The leading theoretical model of dark energy proposes that it is equivalent to vacuum energy – an intrinsic, fundamental energy of empty space across the universe. The energy density remains constant even as space expands. Einstein’s cosmological constant concept elegantly represents vacuum energy.

Quintessence is an Alternative Theory

Some astronomers think dynamic fields like quintessence better match astronomical observations. Quintessence is a hypothesized cosmic scalar field that evolves over time, causing the accelerating expansion. Further observations will help discern whether dark energy is truly constant like vacuum energy or has dynamics like quintessence.

Dark Energy Was Weaker in the Early Universe

Our universe went through phases, transitioning from an early radiation-dominated era to a matter-dominated era. During these phases, dark energy was subdominant to radiation and matter. But about 5 billion years ago, dark energy overtook matter as the dominant constituent as the expansion continued to accelerate.

The Tug-of-War Between Dark Energy and Gravity

On large scales, dark energy’s repulsive gravity counters and overwhelms the attractive gravity of matter. This cosmic tug-of-war is driving galaxies apart from each other. But on small scales gravitation still dominates, which is why individual galaxies remain intact rather than expanding away.

Observations May Reveal Dark Energy’s Nature

By precisely measuring the accelerating expansion over time, astronomers hope to learn whether dark energy is truly constant or evolving. Upcoming surveys like DESI and space telescopes like WFIRST and Euclid aim to constrain dark energy’s properties and help complete our understanding of the substance that dominates the cosmos.

Now that you’ve been illuminated about these two giant mysteries of the universe, maybe you feel inspired to ponder some deep questions: What physics underpins dark matter and dark energy? Why do they exist in the amounts we observe? Do they interact in unseen ways? Answering such questions may lead to breakthroughs as profound as uncovering that 95% of our universe was unknown to us just decades ago. The cosmic dark side harbors many secrets left to still uncover!