Photons are particles of light that travel at the speed of light in a vacuum. According to our current understanding of physics, photons are massless particles, which means they always travel at the speed of light (denoted by the symbol 'c') and never accelerate from a lower speed.
In the theory of special relativity, the speed of light in a vacuum is considered to be a fundamental constant of nature. It is the maximum speed at which information or energy can travel through space. Any particle with mass would require an infinite amount of energy to reach the speed of light, so only massless particles, such as photons, can travel at that speed.
Therefore, when a photon is emitted or created, it instantly travels at the speed of light. It does not accelerate from a lower speed because it has no mass and is always moving at the maximum speed allowed by the laws of physics.