Lambda-CDM model

The universe is very big. It started with a Big Bang. Since then, space has been growing.

Space is full of things. Most of it is hidden. We call this dark matter. It helps pull things together.

There is also dark energy. This pushes space apart. It makes the universe grow faster.

We can see stars and planets. These are made of normal stuff. But this is only a tiny part.

Scientists use math to study it all. They want to know how it works. It is a big mystery!

Scientists use a math model to study the universe. This is called the Lambda-CDM model. It helps explain how the universe works. The model has three main parts.

First, there is ordinary matter. This is the stuff we see. It makes up stars, planets, and gas. This is only about 5% of the universe.

Second, there is cold dark matter. We cannot see it. It does not give off light. But it has gravity. This gravity helps pull galaxies together. It makes up about 26.5% of the universe.

Third, there is dark energy. We call this the cosmological constant. It is a force in empty space. It pushes space apart. This makes the universe grow faster and faster. Dark energy is a huge part of the universe. It is about 68.3% of everything.

The model also includes the Big Bang. This was the start of everything. Right after the start, space grew very fast. We call this cosmic inflation. This model helps us understand the cosmic microwave background. This is a faint glow left over from the early universe. It helps us see how the universe grew.

Scientists use a special math tool to understand our universe. This tool is called the Lambda-CDM model. It helps us explain how everything began and how it grows. The model is the current standard for studying the Big Bang. It works well because it explains many different things at once. It explains how galaxies are spread out across space. It also explains why we see certain amounts of hydrogen and helium. This model is very important for understanding the history of everything.

The model works by looking at three main parts of the universe. The first part is ordinary matter, which is the stuff we can see. This includes atoms, stars, and all the planets. The second part is cold dark matter. We cannot see this matter because it does not give off light. However, it has gravity that pulls things together. The third part is dark energy, also called the cosmological constant. This is a force in empty space that pushes the universe apart. It makes the expansion of space speed up over time.

This model became the main way to study space in the late 1990s. Before this, scientists had many different ideas that did not match. They could not agree on what made up the energy in the universe. The Lambda-CDM model brought these ideas together into one clear picture. It is often called a concordance cosmology because it fits so many observations. Scientists have used it for decades to check their work. Even now, researchers look for new ways to test it.

There are many specific numbers that make this model work. Ordinary matter makes up only about 4.9% of the universe. Cold dark matter is much larger, making up about 26.5% of the total. The biggest part is dark energy, which is about 68.3%. This number comes from data like the 2018 Dark Energy Survey. The model also describes the very early universe. It says the universe started with a Big Bang. Right after, a fast growth called inflation happened within 10 to the power of negative 29 seconds.

You can think of the universe like a growing balloon. The ordinary matter and dark matter are like small bits inside. These bits try to pull together using gravity. Dark energy acts like a pump that blows more air into the balloon. This makes the surface stretch out faster and faster. This is just like how we see distant galaxies moving away from us. The model helps us map this huge, stretching path through time. It connects what we see in telescopes to the math of physics.

The Lambda-CDM model is the primary mathematical framework used to describe the history and structure of our universe. It is often called the standard model of Big Bang cosmology. This model is essential because it provides a consistent explanation for many different astronomical observations. It accounts for the existence of the cosmic microwave background, which is leftover radiation from the early universe. It also explains how galaxies are distributed across space and the specific amounts of hydrogen, helium, and lithium found in the cosmos. By using this model, scientists can map the evolution of the universe from its earliest moments to the present day.

The model relies on three fundamental postulates regarding the nature of spacetime. First, it follows the cosmological principle, which states that the universe is the same everywhere and in all directions. Second, it uses a postulate by Hermann Weyl, suggesting that lines of spacetime intersect at only one point where time can be synchronized. Third, it relies on general relativity, which is the theory relating the geometry of spacetime to the distribution of matter and energy. These principles allow scientists to use the Friedmann equations. These equations describe how the scale factor of the universe changes based on the density and pressure of its contents.

The Lambda-CDM model is defined by three major components that shape the universe. The first is ordinary matter, also known as baryons. This includes the atoms, gas, and plasma that form stars and planets. The second component is cold dark matter, or CDM. This is a type of matter that does not interact with light and moves slowly compared to the speed of light. The third component is the cosmological constant, represented by the Greek letter Lambda (Λ). This represents dark energy, a force associated with the vacuum of empty space that causes the expansion of the universe to accelerate.

In the late 1990s, this model emerged as a "concordance cosmology." Before this time, different observations of the universe seemed to contradict one another. Scientists could not agree on the makeup of the universe's energy density. The Lambda-CDM model resolved these conflicts by bringing disparate data together into one cohesive system. Since its emergence, it has successfully modeled a wide collection of astronomical observations over several decades. While it is the standard, some remaining scientific issues continue to challenge its assumptions and lead to new alternative models.

Specific measurements allow us to understand the exact proportions of the universe's contents. According to 2018 data from the Planck satellite and the Dark Energy Survey, dark energy makes up about 68.3% of the mass-energy density. Cold dark matter accounts for approximately 26.5% of the total density. Ordinary matter is much rarer, comprising only about 4.9% of the universe. Interestingly, most of this ordinary matter is actually unseen. Visible stars and gas account for less than 10% of the total ordinary matter contribution. These precise numbers are vital for calculating the expansion history of the cosmos.

The history of the universe begins with the Big Bang, an event where spacetime abruptly appeared. This was not an explosion, but a sudden expansion containing radiation at temperatures around 10^15 K. Immediately following this, within 10^-29 seconds, a process called cosmic inflation occurred. This was an extremely rapid expansion that multiplied the scale of the universe by a factor of 10^27 or more. This inflation explains the tiny irregularities, or perturbations, seen in the cosmic microwave background. These perturbations are the seeds that eventually grew into the large-scale structures of galaxies we see today.

Understanding the Lambda-CDM model requires looking at how the universe expands over time. This expansion is measured by a dimensionless scale factor. As space expands, light from distant galaxies undergoes a Doppler shift known as redshift. This shift occurs because the light travels across expanding space, changing its wavelength. This expansion increases the distance between objects that are not held together by gravity. However, this expansion does not increase the size of individual objects like galaxies. The model helps scientists calculate the age of the universe and the rate at which it continues to grow.