```ts
const isValidCelsius = (celsius: number): boolean => {
  if (typeof celsius !== "number" || !Number.isFinite(celsius)) {
    throw new Error("celsius must be a finite number");
  }
  return celsius >= -273.15;
};

export function celsiusToFahrenheit(celsius: number): number {
  if (!isValidCelsius(celsius)) {
    throw new Error("celsius must be at or above absolute zero");
  }
  return celsius * 1.8 + 32;
}

export function celsiusToKelvin(celsius: number): number {
  if (!isValidCelsius(celsius)) {
    throw new Error("celsius must be at or above absolute zero");
  }
  return celsius + 273.15;
}
```