Given that this video by Myelin Games popped into my sub box earlier today, I thought it'd be a nice time to add some details + speculation about the source(s) of Light to the pile.
So, the Destiny website's description of the European Dead Zone includes the following:
European Dead Zone was ground-zero for a cataclysmic event that left a massive shard of The Traveler — the source of every Guardian's power and immortality — embedded into the vast landscape.
This heavily implies that the Traveler is the sole source of every Guardian's Light, as a Guardian's "power" comes from their Light-based abilities, and their "immortality" comes from Ghosts. However, there have been arguments made that say the subclasses released with The Taken King- the Stormcaller, Sunbreaker, and Nightstalker- were "created" with Light not bestowed by the Traveler. As Myelin says:
The Taken King subclasses[...] had you unlock abilities from different areas: we harnessed the Void for the Nightstalker from the Black Garden, the Solar energy for the Sunbreaker from the Forge on Mercury, and we harnessed Arc energy from the Arc storms on Mars for the Stormcaller.
This is where I begin to disagree with the theory that there are several sources of Light. We do utilize different forms of energy in the subclass quests in order to 'unlock' new subclasses, but this theory implies that all Arc, Solar, and Void energy is tied to the Light, and that simply gathering energy can lead to new Light-based abilities. However, the Arc, Solar, and Void cards all begin the same way:
The universe is defined by fundamental forces.
Additionally, we see enemy races using these energy types: shock daggers, line rifles, shredders, etc. These weapons are not inherently 'Light,' they simply make use of energy types. As a note, "Energy" itself may be a Guardian concept:
In Guardian parlance, these weapons deal Solar damage.
So, I don't think that Guardians gathering energy from different sources means that they're also drawing Light from these same places. Instead, I'd argue that they're simply taking that energy and combining it with their already-extant Light, the same Light that the Traveler bestows. The Nightstalker's Trail implies that learning how to Nightstalk affects a Guardian's Light itself:
Your light gets twisted. Changed. You find the power to punch through and borrow something from the other side. The Void opens up a hole, and draws from the deep.
While the subclass quests lean heavily on gathering elemental energy, that energy still needs Light in order to create the subclasses. Additionally, subclasses like the Voidwalker have always "drawn from" the Void, so I don't think the wording of the Nightstalker cards implies anything about different Light sources:
Thus we know that the void is full of power. Thus we enter the void without fear.
In conclusion: I think that all Light comes from the Traveler, and that Guardians have learned to combine Light & elemental energy in different ways to form different subclasses. Places like the Forge & the Arc storms are simply places that allow Guardians to combine their Light with elemental energy, enabling them to 'access' new subclasses.
Thanks for reading! This is my first actual post, so sorry if anything's wonky.